Ah, of course, forgot about that.
Enabling $development = true first increased the problems.
To begin with, there was a problem with the permissions of the error_logs folder. Probably my bad, all the folders in xerteonlinetoolkits that I linked as a volume in Docker had admin:users as owner. I fixed that by attaching as root to the container and:
cd xerteonlinetoolkits
chown -R www-data:www-data error_logs
chmod -R g+w error_logs
With that fixed, the browser showed a number of PHP errors:
Deprecated: Array and string offset access syntax with curly braces is deprecated
and
Warning: strpos(): Empty needle
errors in functions.php
(and some headers / session related things, but they were fixed by fixing the ones above).
It may have to do with me using PHP 7.4 (the deprecation one). Anyway, I fixed those and then was able to see a grouping related error:
Warning: Invalid argument supplied for foreach() in /var/www/html/xerteonlinetoolkits/website_code/php/management/management_library.php on line 61
In itself not so helpful, but debug.log showed that it did not like the SQL statement around the word grouping. I do not use database prefixes, if you do you probably won't have a problem there.
I fixed it the way I know that phpMyadmin does: add ` ` around the table name (including the optional prefix).
I had to change that in management_library.php, new_grouping.php, remove_grouping.php to fix the management.php part
I had to fix it in modules/xerte/edithtml.php to get it to work in the editor.
I forked the 3.8.5 code, made the changes in the fork and created a pull request for the changes. If I try to link directly to the pull request, the forum doesn't like my reply.
Leaving $development = true still showed some problems while opening the more complex courses that I imported from Deltion (not with the small ones that I created myself), causing the editor not to open. But the appear to more template related and not grouping related and the went away when I disabled development, so for now I am going to ignore those.