No idea yet what causes it, but this is the exception:

[exception] [cad62ee07fc9bac2866fda42] /index.php/Special:Categories Wikimedia\Assert\ParameterAssertionException from line 63 of /srv/http/archwiki/public/vendor/wikimedia/assert/src/Assert.php: Bad value for parameter $dbkey: should not be empty
#0 /srv/http/archwiki/public/includes/title/TitleValue.php(83): Wikimedia\Assert\Assert::parameter(boolean, string, string)
#1 /srv/http/archwiki/public/includes/specials/pagers/CategoryPager.php(77): TitleValue->__construct(integer, string)
#2 /srv/http/archwiki/public/includes/specials/SpecialCategories.php(56): CategoryPager->getBody()
#3 /srv/http/archwiki/public/includes/specialpage/SpecialPage.php(522): SpecialCategories->execute(NULL)
#4 /srv/http/archwiki/public/includes/specialpage/SpecialPageFactory.php(578): SpecialPage->run(NULL)
#5 /srv/http/archwiki/public/includes/MediaWiki.php(287): SpecialPageFactory::executePath(Title, RequestContext)
#6 /srv/http/archwiki/public/includes/MediaWiki.php(851): MediaWiki->performRequest()
#7 /srv/http/archwiki/public/includes/MediaWiki.php(523): MediaWiki->main()
#8 /srv/http/archwiki/public/index.php(43): MediaWiki->run()
#9 {main}

Triggered by a category entry that has an empty title. I'll look into this again later since I'm not sure what might happen if I remove it or where it could even come from.

MariaDB [archwiki]> select * from category where cat_title = '' limit 5;
+--------+-----------+-----------+-------------+-----------+
| cat_id | cat_title | cat_pages | cat_subcats | cat_files |
+--------+-----------+-----------+-------------+-----------+
| 3412 | | 0 | 0 | 0 |
+--------+-----------+-----------+-------------+-----------+
1 row in set (0.00 sec)

There are discussions about this error on the wiki:
https://wiki.archlinux.org/index.php/ArchWiki_talk:Maintainers#Internal_error_while_accessing_Special:Categories
https://wiki.archlinux.org/index.php/Talk:Init#Subpages

I think you can safely remove the row - you might even purge the whole table and run the populateCategory.php script [1] afterwards.

There are also other invalid rows in the table, because e.g. the list in [2] should be empty (all of the categories listed there have 0 members, but the wiki does not think so). Could you delete the corresponding rows (most likely with "_" instead of spaces in the title) as well?

[1] https://www.mediawiki.org/wiki/Manual:PopulateCategory.php
[2] https://wiki.archlinux.org/index.php/Special:WantedCategories

I've deleted all rows from the category table and recreated it with populateCategory.php. The pages work again and the counts seem ok (non-zero). Can you check if there are any more issues with categories or if all of the ones you saw are fixed now?

The links on the WantedCategories page [2] are still wrong, but now I found that it's probably due to entries in the categorylinks table [3]. There is also a script to rebuild this table [4], though it might take some time to complete.

[3] https://www.mediawiki.org/wiki/Manual:Categorylinks_table
[4] https://www.mediawiki.org/wiki/Manual:RefreshLinks.php

Thanks! The refreshLinks.php script removed some things and the WantedCategories page is now empty. Is that the way it's supposed to be?

Thanks for the quick update! I think all the errors we noticed are solved now.