Figured out i parted out the wrong Philips SL18. I had two: one worked fine, the other produced a horrible burnt electronics stench.
I had screwed my 'good' one into a socket and after a while... stench. Ugh.
I've now opened up the other one, but i can't really see anything wrong with it. It just runs very hot.
I kept the ballast of the other SL18 so i could conceivably swap them out. But it's hard to get access to it, without damaging the E27 socket. Either way, it's now running without the glass prismatic cover, so i can get an impression of what is running so hot - the ballast or the tube itself. The glass cover would get so hot, that i couldn't hold it for longer than like 5 seconds. That's not normal for an SL...
Might have to build a ring tester to see if the stinky one has a shorted out winding in the ballast. I hope not. One thing's for sure - i can see that the capacitor is undamaged.