I have same problem. I can't login as regular user also as admin if WPFORO plugin is enabled. If I WPFORO disable user's login works well.
I use Wordpress 5.7 and WPFORO 1.9.6.
My webpage is www.KurzExcel.sk, now the WPFORO is disabled because of login user issues. I have no installed cache plugin.
You have several errors and issues (js and possibly others) at your site. Press F12 and check your browser console.
This is a Wordpress and not wpForo issue, since logins are handled by Wordpress and not wpForo.
This could be plugin conflict as well. I'd recommend activate wpForo then disable other plugins one by one to find the one which hooks the login process and stops it.
If you leave a screenshot of the problem or the error message, it would be helpful too.