A couple members have reported getting this error message (below) when sending PM's to a certain member:

Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator, []webmaster@mysite[/] and inform them of the time the error occurred, and anything you might have done that may have caused the error.

More information about this error may be available in the server error log.

Any ideas what this might be? It appears that this lone member has this problem.

Have you access to your apache(?) error log? It should provide some details about the error.

Yeah, those look like server errors and not really threads errors.