I received a 500 error page while attempting to access the forum page… This means there is also a problem outside your group page and/or a server misconfiguration.
What type of server do you use ? Shared or VPN or… ?
What are your permalinks settings ?
What kind of htaccess do you use ?
To handle what Paul said, try to reload all BP files via FTP and see if things changed. If not, you have to debug trought your plugins and check above mentionned points.