There is a known Stack Trace error in UBB.threads 7.7.0 and 7.7.1 when running PHP 7.1 (or newer). The Stack Trace errors will not occur if you are using PHP 5.4 through PHP 7.0.
This has been patched for UBB.threads 7.7.2, set to be released in March 2019.
The error only occurs when your forum is set to require admin approval for new user registrations. And only occurs at the time of sending the "Your account needs to be approved by an Administrator before you can log in. You will receive an email once this has been done" email. The account is still created and can be activated by an admin, except the registering user sees an error page, and does not receive that notification email.
That patch consists of correcting 1 line in 1 file, which is:
in scripts/adduser.inc.php
FIND NEAR LINE 762
$mailer->add_content('REG_CONTENT5');
REPLACE WITH
$mailer->add_content('REG_CONTENT5', null);
--
Within that same file there are 2 additional lines to patch, which are semi-related to your reported bug. They patch a possible Stack Trace error regarding the "Notify me when a new user registers on the forum" notifications. ie, the "There is no approval required, but you have chosen to be notified of all new user registrations" email which is sent to admins which request it. This is also a PHP 7.1 (or newer) issue.
Those two lines are:
also in scripts/adduser.inc.php
FIND NEAR LINE 800
$mailer->add_content('REGN_CONTENT1');
REPLACE WITH
$mailer->add_content('REGN_CONTENT1', null);
also in scripts/adduser.inc.php
FIND NEAR LINE 803
$mailer->add_content('REGN_CONTENT3');
REPLACE WITH
$mailer->add_content('REGN_CONTENT3', null);
