@Ronald i got same error message
Error: Call to undefined method IPS\arcade\extensions\core\Notifications\arcade::configurationOptions() (0)
#0 /home/oldlrhvz/public_html/ipb/system/Dispatcher/Controller.php(90): IPS\core\modules\admin\membersettings\_notifications->edit()
#1 /home/oldlrhvz/public_html/ipb/applications/core/modules/admin/membersettings/notifications.php(38): IPS\Dispatcher\_Controller->execute()
#2 /home/oldlrhvz/public_html/ipb/system/Dispatcher/Dispatcher.php(153): IPS\core\modules\admin\membersettings\_notifications->execute()
#3 /home/oldlrhvz/public_html/ipb/admin/index.php(13): IPS\_Dispatcher->run()
#4 {main}
I can save all notifications (edit) without problem, but not arcade notifications.
GMP PHP extension think has nothing to do with that error