I expect, that you have enough experience with MySQL/MariaDB to alter the column definition for mlf2_useronline.ip yourself from the length of 15 to 128 characters. Please report, if this changes anything.

YES!
Here is a Google translated version of a text, I have written for the users of my forum:

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The error has been found!
Originally, an IP number was a maximum of 15 characters, e.g. 12.128.230.100.
In the latest version (IPv6), the number is significantly longer, e.g. 2a03:8108:1200:4f78:37bd:163:b3f6:41df.
Far down in the forum's database is a table called "useronline", which contains the IP number and time of all users who are online in the forum.
The length of this field was set to 15 characters and when I changed it to 128 - bingo! Then the forum worked again for the users who previously got the error "HTTP error 500".
This error has always existed so why it only activates when I switch from PHP 8.0 to PHP 8.1 is unknown. Perhaps PHP 8.0 simply ignored the error.
It was also the same error that blocked a number of users in the Signalposten.dk forum, and it has now also been corrected.
Anyways - I hope everyone is back, and I´m sorry for the downtime. Thanks to everyone who contributed with error messages, solution suggestions, etc.

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Thx for finding this pesky bugger. 🙂

/Tommy