I have a few members that are unable to send or reply to PMs. Their usernames only contain letters (no spaces, etc).
The one member in particular that I am working with to resolve the issue is a moderator - she has told me that she gets a 500 internal server error whenever she tries to send or reply.
I am running version 5.47e with various hacks installed. I have been running PM3.8 for awhile, but have just not had time to fix this issue. It works great for all but about three members (we have 600 members).