I tried another laptop now, newer, more RAM, and it is working.
The old one still doesn't work, althougt it was working until January and now has 2 GB RAM open for Java.
So there is definitifely an issue, for me still fixed now.
I got 7 crash reports, out of which 4 were caused by the tesselation bug, 2 null pointer exceptions and one memory overflow.
Since in 19w08b the problem until now seems to be gone, i guess the problem was solved with the tesselation bug and i don't have to upload neither world nor crash reports here.
Nonetheless thank you for your fast response, of course i knew what i wrote wouldn't be enough, but i didn't have the world at hand there.
Still thanks a lot!
When being cured, the villager instantly chooses a correct profession now, but still does not change it later when the work block is removed and another one is placed.