I'm working on the ticket right now, but to be fair, according to your system log it's def. a 3rd party issue:
ArgumentCountError: Too few arguments to function IPS\core\modules\front\system\_register::_createMember(), 2 passed in /var/www/html/105119/init.php(893) : eval()'d code on line 26 and exactly 4 expected (0)
#0 /var/www/html/105119/init.php(893) : eval()'d code(26): IPS\core\modules\front\system\_register::_createMember(Array, Array)
#1 /var/www/html/105119/init.php(893) : eval()'d code(90): IPS\core\modules\front\system\hook1905::_createMember(Array, Array, NULL, Object(IPS\Helpers\Form))
#2 /var/www/html/105119/init.php(893) : eval()'d code(15): IPS\core\modules\front\system\hook2764::_createMember(Array, Array, NULL, Object(IPS\Helpers\Form))
#3 /var/www/html/105119/applications/core/modules/front/system/register.php(240): IPS\core\modules\front\system\hook2898::_createMember(Array, Array, NULL, Object(IPS\Helpers\Form))
#4 /var/www/html/105119/applications/core/modules/front/system/register.php(174): IPS\core\modules\front\system\_register->_registrationForm(NULL)
#5 /var/www/html/105119/system/Helpers/Wizard/Wizard.php(181): IPS\core\modules\front\system\_register->IPS\core\modules\front\system\{closure}(Array)
#6 /var/www/html/105119/applications/core/modules/front/system/register.php(181): IPS\Helpers\_Wizard->__toString()
#7 /var/www/html/105119/init.php(893) : eval()'d code(27): IPS\core\modules\front\system\_register->manage()
#8 /var/www/html/105119/system/Dispatcher/Controller.php(101): IPS\core\modules\front\system\nexus_hook_register->manage()
#9 /var/www/html/105119/system/Dispatcher/Dispatcher.php(152): IPS\Dispatcher\_Controller->execute()
#10 /var/www/html/105119/index.php(13): IPS\_Dispatcher->run()
#11 {main}
This error means literally that a plugin or application hooking the register::_createMember method is failing because it isn't compatible with IPS 4.5