It says your server doesn't support PHP email sending. Its server issue. You should contact to hosting service and ask them enable email sending functions of your server. This is not only wpForo issue, I guess the WordPress is not able to send any email too.

Posted by: cgarst

 

Yes, there is an error log there, however no errors are in it for a long time.

   

Install POSTMAN plugin. That plugin provides extended email logs and also a TEST email function. You will find the cause.