500 internal server error

For some time now i cant get rid of this error. I searched every site on the internet but nothing worked.

Every time someone fills the form and press send 500 internal server error appears.

This is the line from the error log:

PHP Fatal error: Only variables can be passed by reference in /home/energeti/public_html/wp-includes/class-phpmailer.php on line 1104

I have double checked every input in vfb settings and also wp settings. Asked if its something wrong on the server side but everything seems to be ok.

https://www.ads-software.com/plugins/visual-form-builder/