This instance of the issue was fixed/workarounded via this edit.

I firstly tried to see the status of task pool via https://da.wikipedia.org/wiki/Speciel:NewcomerTasksInfo, which throws an error (filled as T279651). Then I logged into mwmaint1002 to see if listTaskCounts.php has the same result, and I found out it does not:

[urbanecm@mwmaint1002 ~]$ mwscript extensions/GrowthExperiments/maintenance/listTaskCounts.php --wiki=dawiki
Search request is longer than the maximum allowed length. (Actual: 465, allowed: 300, not counting articletopic)
[urbanecm@mwmaint1002 ~]$

This immediately led me to look into the history of MediaWiki:NewcomerTasks.json, and I noticed there were a lot of templates added by an on-wiki administrator. The templates exceeded the limit for search query imposed by the API, which means the API errored out. As a result, the task pool was empty.

I will send a message to the administrator who added the templates, and discuss what to do next with them. One possibility is to use child templates – the templates they added appear to all use Template.standardstub. We can add that to the list of templates, which will still work, while keeping the query under-the-limit.

Moving into sprint and QA: @Etonkovidova I think we should make sure the same issue (empty task pool for a topic) doesn't happen on other wikis, or with other topics. It should be possible to check that via loading https://da.wikipedia.org/wiki/Speciel:NewcomerTasksInfo on our target wikis.

Posted a message on the administrator's talk page: https://da.wikipedia.org/w/index.php?title=Brugerdiskussion:InsaneHacker&diff=10700736&oldid=10660857.