OK first of all this is kind of similar to this thread, but I tried to clear the cache but it didn't work.
But it's not the same problem, because if I turn Oxygene theme and log out and back again Lancelot is rendered perfectly, but if I turn my theme back or some other themes the problem with Lancelot returns.
Lancelot with Oxygen theme:



Lancelot with my theme:



Now how could I get my theme and Lancelot to play nice??

What version of KDE are you using? If it is not newer than 4.1.96 (or to be more precise newer than 2009-01-16) than it probably is that /other/ problem (the one in the other thread). And as you can see, some SVGs are screwed in the Oxygen screenshot too (the system buttons)

Yeah I guess it's really the other problem...
I have 4.1.96 and I haven't noticed the system buttons are screwed in Oxygen to...
Will mark this as solved and act accordingly to the other post...