I did a test: When my computer is off, the Osram LED filament at the corridor of my ceiling at my hostel, working without a problem, but when my computer is on, it vibrates in brightness.
Why?
https://www.youtube.com/watch?v=4BnWg9XQ9dE

I cannot see it in the video.

Is it behind a dimmer? Dimmers are very sensitive to other devices on the grid causing interference.

Your computer may be causing that interference. In earlier days people would quickly complain about that because it makes AM reception impossible, but today it can remain undetected until things started acting weird. This is also the reason why fluorescent fixtures are required to have suppression capacitors (at least in europe).

If it is not just your computer causing this but the issue also shows up when you connect a 300-500w incandescent lamp or a heater on the outlet where you normally plug in your computer, and your lamp starts flickering randomly, this means there is DANGER. With the small load of the lamp there is no problem, but once extra load is turned on, a wire nut or other mains voltage connection starts to heat up and spark, causing flickering. This is very dangerous.

The computer is the cause, as it don't shimmering when the computer is off. The voltage on that phase, is usually 200-210V.

If the voltage on that phase there is just 200..210V, computer is not the dause, faulty wiring is. The fault cause the PFC in thecomputer to be unstable, causing voltage amplitude to getmodulated by the instability and so the lamp starts to flicker.
So better ispect the wiring, such low voltage on a 220V, moreover 230 or 240V rated mains is everything but normal...

Modern PSUs are rated for 100-240V, 50-60hz.

They are, but are also rated only to work from a low impedance voltage source, so from a source where the voltage does not change with load change.
If you add a series impedance, the reduced voltage is by far not the only thing the circuit has to deal with, the fact the voltage droops when the PSU sucks more current is.

In a steady state power factor correction operation, the internal regulation loop needs to be slow, so it won't respond to the AC voltage (the sinewave), so the input current can follow the input voltage shape, so it won't cause distortion. But this creates a problem when the conditions, mainly the output load, it has to have extra mechanisms to respond quickly to prevent undervoltage or overvoltage on the DC filtering capacitor. Then the circuit settles into a new steady state operating point, where again the regulator responds rather slowly and so maintains low distortion. The over/undervoltage actions do lead to anything but a sinewave input current, but that is not that big deal, as it is only active during big transitions.

The problem is, when the supply source is weaker, has high impedance. When these under/overvoltage functions activate, the current surges/dips cause big change of the input voltage but mainly do not lead to sufficient recovery effect, so the controller goes into more severe action, so larger current surge and larger dip. And the whole thing starts to oscillate, at some fraction of the mains frequency, so some 1..30Hz.

It is technically possible to design a PSU able to work on weaker input power source (it mainly needs larger DC tank capacitor after the PFC and mainly extra controller functionality so it is able to find a stable operating point), but normally there is no reason for such extra complications for mains powered PSU.
So when the supply on a standard mains equipment becomes weak, these instabilities are quite normal, often triggering protection circuits or even not so rare is causing excursions leading to some component destruction (good example are cheepeese inverter welders operated on small gensets - these combinations tend to fry either welders, generators, damage the engine or any combination of them).

I forgot to say that my computer is connected to a Line Interactive UPS with an AVR which boost the voltage when it is low, so now my computer don't gets the low voltage of the phase it is connected. So why the LED lamps still shimmering?
My computer PSU is Seasonic 620W Active PFC 80+ Bronze.

The boosting UPS fixes just the steady voltage problem. It won't have any effect on the high impedance.
Even worse, the flickering could actually come fromthe UPS, when that is searching for a good boosting ratio, when the computer power supply (a constant power input, so higher voltage means lower current) may cause the UPS to overshoot, so starts "hunting".
Neither thecomputer power supply, nor that UPS are designed for the input power feed tobe so weak the load these things exhibit has impact onthe mains voltage.
They are designed to get the input voltage in a wide range, but it should not change in response to the load these things present.
If the impedance gets so high the mains voltage responds to the load, the regulation may become unstable, so starts oscillating, so the lamp to flicker...

249V output voltage looks strange.


Some years ago I did a repair on a large Furman voltage regulator/stabilizer, intended for on-stage application. The unit generally worked, but output voltage jumped chaotically and indeed it caused some strong interference and light flickering on the mains upstream. Regulator topology was a large toroidal autotransformer, very heavy, circa 3kVA, with about 15 taps. Taps were selected by a series of optocoupler controlled TRIACs, switched by a digital output of a reversible counter. Control loop idea was when output voltage goes out of spec, counter jumps one step up or down and selects a next tap on the transformer. Somewhat reminiscent of the very old-school idea of a reversible servo motor  rotating a VARIAC brush, but done in semiconductor implementation. I spent may be a half-day and the evening reverse-engineering (these jerks do not provide schematics) and looking for an actual defect. Long story short, an idiot who designed this professional and rather expensive unit cheapened to use a base-emitter junction of a general purpose silicon transistor as circa 7V main reference voltage zener diode!  Over the time this 'zener' degraded and become noisy and jumpy, causing transformer taps to be changed in a fast chaotic way, out of sync to line periods, and actually generating intense voltage sags and interference on the  power line!

But clearly this UPS is controlled by the microcontroller, which also measures the output voltage. And I doubt it would use separate voltage sensing for regulation and separate for displaying the voltages in the app. Most likely it uses a common V-meter (I mean divider, ADC and processing) for both displaying the output voltage, as well as controlling the transformer tap switches.
So if it displays 250V, it means either that is what the regulation aims for (within the tolerance), or the regulation loop itself got crazy (due to input feed impedance and load behavior, a combination the thing was apparently not designed for) and it is just hunting around to reach the correct output so much, the reading after averaging (i guess the regulation uses fast ADC readouts, but the display averages/filters them out to get somewhat stable reading on the "display") it shows the voltage being higher than actually intended in average.

And essentially the same principal functionality (minus the user readout display) is in the PFC of the computer PSU. So you have a cascade of two elements which both could get crazy under some not so usual conditions, so making the combination even way more likely to go crazy...

But it could also be fault in one of them. The computer PFC very likely uses analog regulation loop, so will use analog capacitors and resistors to program the dynamic response behavior. And the main power tank capacitor is also part of this loop. So once some of them dries out and degrades, the parameters may shift so it starts become unstable by itself, or at least more sensitive for higher mains impedance (and there the boosted voltage from the UPS will have higher impedance than the direct mains when boosting, due to the extra resistance of the transformer winding, even when the UPS itself is behaving correctly). And the computer PSU oscillations may then also upset back the UPS control, throwing everything into the deep mess of unstable regulation loops.

I doubt the control problem could be in the UPS itself, as there all the dynamics is most likely implemented in the processor, so not relying on any parameter of external timing components, except of the CPU clock generation. And because this clock generation is also used for the communication and the communication to the external world is working, the CPU clock will not be defective.
The only thing that remains is some faulty switching element on some regulation step. Then the voltage would disappear completely instead of e.g. getting reduced by a few volts. But on the other way I would expect the firmware will contain some detections of such "impossible" states (the voltage dropped to zero instead of just by few V) and trip some alarm and/or safe foldback mode.