Hi, and welome :)
That is a bug and not your mistake. PHP changed what can be done with references, and it seems the adduser plugin is not sufficiently often used with modern PHP versions to have that bug already be reported.
There is a fix: Change line 17 of plugins/serendipity_plugin_adduser/common.inc.php from
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static function sendMail(&$username, &$hash, &$email, $approve_only = false, $admin_cc = true) {
to
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static function sendMail($username, $hash, $email, $approve_only = false, $admin_cc = true) {
(remove the &). I did not test the functionality of the plugin after that change - I'm not familiar with it - but from looking at the code I think this should change nothing, apart from making it work ;)
I will push a fix to spartacus, but you will sadly have to do the change manually to make your blog working again, since you can't reach spartacus like that.