I have experienced this problem as well. I use the Caribbean Track and Field forum/http://disc.yourwebapps.com/Indices/113916.

I have been experiencing this since:
Thu August 16 6:20pm
Fri August 17 7:34pm
Sat August 18 9:56am

Once I access a subject, I can reply but when I click on back to messages. I get this:

Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.

Please contact the server administrator, webmaster@yourwebapps.com 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.