We are using the minimum version of sphinx when building the documentation:

This probably a good thing when the CI temporarily fails and it gives us time to adapt our build script. However, now we are pinning sphinx to 3.2.0 while sphinx is about to release 4.0. Thus, we are lagging quite behind without any clue regarding the build issue.

I would advocate using 'latest' for the doc build whenever possible and if we need to pin the version to add a comment with the reason with an attached issue such that we can keep track of the problem.