So... have done an upgrading 3.3.0 to 3.3.8, it's working good but I found an issue regarding memberlist. Hitting the send-button ("Skickat") gives me an empty page (PHP-fault?).
I've tried to see what memberlist.php is supposed to do but I'm stucked here. The phpcodechecker.com gives a warning on an array-statement but it's not an error.
Your style we_universal doesn't seem to be uptodate.
And I suspect your se- language pack isn't either.
Spelling is freeware, which means you can use it for free.
On the other hand, it is not open source, which means you cannot change it or publish it in a modified form.
Time flies like an arrow, but fruit flies like a banana.
we_universal is not supported anymore and as I see it's only in html-code, not any PHP. Should not affect this... I think.
So - I've upgraded phpBB to 3.3.9 and the one and only plugin is also up to date. So the language pack (not changed since 2015).
The problem is still there, the memberlist function is working with PHP 7.4 but not with PHP 8.1 (the result is a blank page).
In PHP 8.1 the URL is: ./adm/index.php?i=acp_users&sid=dd391677a445d2bf5648bb72430d97a9&icat=13&mode=overview
In PHP 7.4 the URL is: ./adm/index.php?i=acp_users&sid=e3c35df832875d2de09b6841a3107a6d&mode=overview
Note: I'm running 8.1 in a production server, the 7.4 runs in a test server (the files in the test server are an image of the production server, just some data changes related to the database SQL access is made).