More than likely the thread was lost in posts.
What software versions of vbulletin and photopost?
Does this happen when you disable the plugin system?
Miharu Desktop - PhotoPost Photo Gallery
we have a pretty stock vb3 site here with no real plugins other than vbseo and I have tried a couple different sites as well as new vb4.
It really would be a matter of tracking down what settings or plugins etc that might be interfering with things.