[28-Aug-2018 15:14:28 UTC] PHP Warning: Cannot modify header information - headers already sent by (output started at /home/wp-content/plugins/wpforo/wpf-includes/class-subscribes.php:234) in /home/wp-includes/pluggable.php on line 1219
I use no plugins for users or registration-related or anything, just Wordpress.
This error says you have PHP error in your website, that php error can be generated by theme or other plugin. It has no direct relation to wpForo. When you click on confirm subscription link, wpForo does some job then calls wp_redirect() function. This is WordPress core function and it can't redirect if there is some error in buffer. So you see "PHP Warning: Cannot modify header information - headers already sent" error.
In other words you have PHP errors in website, which cause this error (cannot modify header information) as a result. Once you fix the main errors, this error will be fixed automatically.
Try to find error.log files and find other error messages.
Well i don't have any php error in my site. None.
Disabled all plugins, still the same error.
Strange. It happens only to a site located on a subdomain.