Well, I upgraded vBulletin to 3.0.10. After that I upgraded Photopost to 5.21.
After that the photopostfeature on the forum main page showed only black pics. The url's to which the thumbnails are linked work just fine. If I click a thumbnail, I end up on the right page. But there is no thumbnail to view, it's black!
I reverted back to an older version inc-features.php, and it works fine. Except that I just have one thumbnail, no matter how many I specify in the file. I guess I miss some customizations I apparently did to the previous version and I have no backup of it. And I lack the brainpower to deal with it. Too tired.
In short, I guess vB
3.5 compatibilty is great, but at the cost of compatibility with versions smaller then 3.5?