In the meantime, the best thing appears to be to open the site in a new window/tab rather than trying to use the Back or Refresh buttons on your browser.
If you do encounter the error, I'd be really grateful if you could immediately post in this forum so that I can get an idea as to when this is happening (e.g. if it is happening at a specific time of day).
Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator, 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.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.