Remove Siteground optimizer, clear your browser cache and try again. And it would be better to stay away from SG optimizer and any optimizer (not needed with http2).

@engin,

First, install Really Simple SSL plugin to make sure you always use HTTPS protocol. Install the plugin, go to the plugin setting page and enable the SSL redirection.

Then exclude forum pages from your cache and optimizer plugins. wpForo does it itself and care about the forum cache. For the Siteground optimizer follow the instruction below, if the issue is not fixed just uninstall it, it doesn't help your forum, it stops all dynamic functions and makes your forum as a dead page.

SiteGround Optimizer plugin

1. Please navigate in Dashboard to SG Optimizer > Caching admin page
2. Scroll to Exclude URLs from Caching section and click the "pencil" button, enable it, and click the button again
3. Insert the URL path(s) of your forum page(s) with wildcard [*] in the pop-up filed:
   

   /community/*
   /participant/*
   /sign-up/*
   /sign-in/*
   /change-password/*

4. Save it and delete all caches.

Thank you @robert, @dimalifragis 

Just an update: When clearing the Chrome cache the "all time" option must be selected. Otherwise, the same issue keeps repeating. 

The problem is solved.