UBB.threads PHP Forum Software Community Forums UBB.threads Version 7/8 Support Users can't send PMs. Browser settings??

Ive got a small number of users (2 or 3) who cannot send PMs. They get an "Error in Page" displayed and the PM isn't sent.

Of course, when I login to their accts I can send PMs just fine. This indicates to me that its a browser settings issue.

What are the basic browser settings that are required for the board features to work properly?