As mentioned here by RMcGirr83: https://www.phpbb.com/community/viewtopic.php?f=661&t=2584876

If a user is banned, and debug errors turned on in php.ini, following warnings present themselves

Warning: Undefined array key "session_page" in F:\wamp64\www\qi\boards\phpbb33\phpbb\session.php on line 1663

Warning: Undefined array key "is_registered" in F:\wamp64\www\qi\boards\phpbb33\includes\functions.php on line 4052[/quote]

phpBB 3.3, PHP 8

...and no I won't open a ticket for this.

 
Reproduced on both PHP 8.0 and 7.4. If a user is IP-banned, the above errors are shown.