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...