I try to register a new user for testing my board and email content. After I fill out the form and final submit it I get this error:

Code
Fatal error: Uncaught ArgumentCountError: Too few arguments to function mailer::add_content(), 1 passed in /kunden/166911_12355/rp-hosting/1020/1020/websites/dragonclan-forum.de/board/scripts/adduser.inc.php on line 762 and at least 2 expected in /kunden/166911_12355/rp-hosting/1020/1020/websites/dragonclan-forum.de/board/libs/mailer.inc.php:139 Stack trace: #0 /kunden/166911_12355/rp-hosting/1020/1020/websites/dragonclan-forum.de/board/scripts/adduser.inc.php(762): mailer->add_content('REG_CONTENT5') #1 /kunden/166911_12355/rp-hosting/1020/1020/websites/dragonclan-forum.de/board/ubbthreads.php(279): page_adduser_run() #2 {main} thrown in /kunden/166911_12355/rp-hosting/1020/1020/websites/dragonclan-forum.de/board/libs/mailer.inc.php on line 139

The user was created and I can see him the control panel. The verify email was not send out.
At this point I can press F5 and get back the forum with the info that the user now already exists.
The error musst have something to do with creating the register email.
Mail a user in the control panel and sent out notifys to users works well.

More informations:
I try this while using stock englisch language files
Server ist running php5.6 but I setup a .htaccess file for the ubb.threads directory to run the forum with php7.2.11
Forum is not using any hacks.
I setup the register queue to verify any new user (need approval)
I use StopForumSpam

As I can see there was a modification in line 757 of adduser.inc.php in V7.7.1 (append a ,null). Can cause this the error? Something missing?

my board: http://www.dragonclan-forum.de
my hobby: http://www.biker-reise.de
Ich kann bei Fragen zu UBBthreads in Deutsch weiterhelfen oder es zumindest versuchen