I have no idea. If it is a plugin, disable it. If not, ask your hoster to help you out.

@clarnico This <!-- generated 12 seconds ago generated in 1.122 seconds served from batcache in 0.002 seconds expires in 288 seconds --&...

This one.  Screenshot.png

You have some other plugin(s) messing things up. For example the "suggestion" offer, is not either WP or wpForo message.

@imfhope Your WP theme. If you press F12 in your browser, you will see a console. Errors of the session are displayed in there.

@imfhope Please find a better theme to use, your forum doesn't display properly. It is very very narrow and it will create issue to your users. I see ...