Can you try using a VPN clinent (or browser extension) and report back? The problem is likely originating from your provider and a proxied connection could likely help bypass that.
That solves my problem, but I want to be able to use the forum normally, not using a VPN connection every time. 
Have you checked your firewall? You should check the settings to see if there is any option that blocks ssl.
I also found this:
Changing your internet security setting can be a solution of ERR_SSL_PROTOCOL_ERROR in chrome. To do it-
Go to Control Panel > Network and Internet.
Then click on Internet Option.
Now from the Security tab select Internet and set security level Medium.
From Privacy tab, set privacy level medium too. (If this option is available)
Now restart your computer.
Thanks for the advise, but it didn't solve the problem. I also found this:
Changing your internet security setting can be a solution of ERR_SSL_PROTOCOL_ERROR in chrome. To do it-
Go to Control Panel > Network and Internet.
Then click on Internet Option.
Now from the Security tab select Internet and set security level Medium.
From Privacy tab, set privacy level medium too. (If this option is available)
Now restart your computer.
