It's not with DiscussionTools. You have this problem on every page. If you think it is caused by DiscussionTools, disable it again and compare.

Try to disable https://www.mediawiki.org/wiki/Manual:$wgSend404Code

@Wargo Yes, this problem presents on every Page where the Discussion does not yet exist. Disabling DiscussionTools does resolve the issue: I only noticed the issue after installing DiscussionTools. The testing instance I linked has minimal customization/configuration, only:

# End of automatically generated settings.
# Add more configuration options below.

wfLoadSkin( 'Vector' );
wfLoadExtension( 'VisualEditor' );
wfLoadExtension( 'Linter' );
wfLoadExtension( 'DiscussionTools' );

Adding $wgSend404Code = false as you suggested does also resolve the issue, while preserving DiscussionTools! Can I consider this an acceptable workaround, or does it have bad side effects?

The workaround suggest this is problem with server config but weird that it is only when DT enabled.

Helpful for debugging (for developers):
https://gerrit.wikimedia.org/g/mediawiki/core/+/master/includes/page/Article.php#1611 to 1622, hook used by DT
and function shouldDisplayEmptyState

The config manual says that edit views never get 404s, only read views – and DiscussionTools turns &action=edit&redlink=1 URLs from edit views to read views, likely triggering the configuration error of your web server software. If my analysis is correct, https://pommel.uber.space/index.php?title=Talk:Wiki_issues should display the unfriendly 404 page regardless of whether DiscussionTools is enabled as long as $wgSend404Code true.

In T380885#10363622, @Tacsipacsi wrote:

If my analysis is correct, https://pommel.uber.space/index.php?title=Talk:Wiki_issues should display the unfriendly 404 page regardless of whether DiscussionTools is enabled as long as $wgSend404Code true.

Yes, and non-discussion pages (as I remember) too. Currently he set this to false.

Thank you for your swift responses. Installation on our production server (quite a different environment) was issue free https://wiki.kanthaus.online/Wiki_tasks I will contact the provider https://uberspace.de/en/ and update the guide I wrote for their service https://lab.uberspace.de/guide_mediawiki/