This is happening once again - the COMODO-issued certificate for forum.middlemanapp.com expired on Feb 7th 2017 (two days ago).
middlemanapp.com is using a Let's Encrypt certificate, though - but it isn't a wildcard certificate, so it doesn't work for forum.middlemanapp.com
Google Chrome refuses to connect - Firefox allows you to add a cert exception, which is not really great.
It'd be great to get this fixed - migrate it to Let's Encrypt, I think.
I could totally help you with this, if you want.