Blank page in PM

hi,

I have been using ibProArcade v2.6.6+ on vBulletin 3.6.8 Patch Level 2.
Everything was OK, but yesterday I realized that when user is getting PM about new tournament ("New Tournament in the Arcade") and wanna read it - he cannot do it, because of blank white page communicate. Every other PMs are working fine (for example about users records in games).

edit: one of my moderators sent me that screenshot:

any idea?

Thanks,
Jakub Krezel