Hello

I changed PHP to 5.5 without noticeable change. More is not possible.

Even I must say, that PHP 5.5 also is outdated, I can say that I expected no difference between PHP 5.4 and 5.5 or even 5.6 or 7.x at that point.

I had a look at the PHP config:
sendmail_path : /usr/sbin/smtppush -t -i -f info@mysite.be

No idea what it means.

A program call of smtppush when taking over an e-mail, must be suffixed with the parameters -t -i -f info@mysite.be. As far as I know the e-mail, given in parameter -f must be the sender. For more details ask your hosting company (FAQ, customer service) and compare the forum settings (advanced settings) of your two forums. Look there for the setting mail_parameter. By the way, this is the setting, where you have to store the real parameter list (in your example -t -i -f info@mysite.be).

The server log doesn't help. Disabled functions are : phpinfo, system, exec, passthru, proc_open, shell_exec, popen, setlimit, glob, mysql_pconnect, stream_socket_client"

These functions are not related to your current problem.

Tschö, Auge