It seems a whole load of forums are showing this message today, so it would appear to be some sort of general problem across the forums. ( See http://help.forumotion.com/t154680-ssl-certificate-problem#1061643 )
The reassurance here is that the message is misleading - as all the forums that are finding this message this morning are TOTALLY AND ABSOLUTELY SECURE - they still have the valid SSL Certificate and there is nothing to worry about, the forums can be used as per normal - the tech team are investigating, and let's just hope that whatever has caused this blip can get sorted soon.
(Someone has suggested it might have something to do with the clocks (current date & time) on the forums ....)
well, whatever the cause, they are onto it and we hope to have a solution soon.