I've taken to using the "mark forums read" button at the bottom of the forums main window, which actually works. 8)
Correct me if I'm wrong, but it looks to me like these forums and the curseforge forums are actually the same, just with a different face. It's been a while since I checked, but as I recall, the same error appears there.
Thanks for the tip. I was actually trying to use the link in the "Quick Links" dropdown, and the link you are describing doesn't exist in the "new posts" page. I think that link is some kind of standard one and has been updated when vBulletin has been updated, in contrary to the dropdown one.
I thought it was because I forced some "big" number of minimal font size (12) and default font size (16). But when I remove the minimal font size and set the default to 9 it still happens. Firefox zoom doesn't change it either.
Edit: Having enabled Firebug, I see that the dropdow is positionned using "absolute" style with pixel values. Not sure if and how it was calculated or fixed. However this explains why the font size doesn't change it.