I totally forgot about this one:
The PDF, these screenshots came from, was apparently malformed. While Preview was able to display it, it failed to export any raster image of a page. So I tried to import and render it using Affinity Photo, but the process didn't "come back" and I had to kill the task. Which is, why I then resorted to screenshots.
I would have expected any related temporary items to be purged on application restart, and certainly on system restart. However, it may well be that this hanging task was invoked again instead and was thus hogging the application thread somehow from the background.
So the issue may be related to not cleaning up any temporary items on application (re)start?
(I guess, any temporary resources are also a good candidate for becoming cleaned up in the course of the daily system maintenance tasks, explaining why it worked again today.)