Can't edit pages after upgrade

Hi. I've been using PmWiki for a *very* long time.
Recently, I upgraded to version 2.43 as the previous version was not compatible with the upgraded PHP version on the host.

The site looks normal, although some minor details, such as (:notabs:) is obsolete.
When I try to edit an existing page, the edit textarea is blank. It's as if it's a new page.
I checked folder permissions, which are OK.
If I create a new page, the page is created, but no content is saved.
The file is saved, but the text attribute is empty.

I see this warning in the logs:

PHP Warning: Undefined array key "title" in ...pub/skins/monobook/monobook.php on line 98'

I tried the default PmWiki skin, but that didn't help.
The server is running PHP 8.3.19

Any help would be appreciated.
Thanks!