This is a strange case that happened to me too, but I was able to fix the issue. Turns out for me that after I added the Go Daddy Site Scanner seal to my sub_footer my forums started having this problem. I couldn't open the forums at all just keep getting the red box message saying site is currently not available. After I removed the site scanner seal for my sub_footer my forums started working correctly again. So only advice I can give on this forum error is to check any script you may have added see more to your site or template files because that could be the issue.