I've been building Interfaces ever since, and I never had this problem until the present day.
3 days ago, my graphic card and RAM died, so I bought a new RAM (4GB) and a Card (Ati Radeon HD5720, if it has anything to do with the problem).
I took the opportunity and thought I'd clean up my UI a bit, and ended up redoing everything, including re-downloading all addons.
As you can see, several borders are somewhat fuzzy and not sharp as I'd like them to be. I stumbled upon the /-command you can see in the chat in the forum, but somehow, it didn't really help to solve my problem.