Help with debugging weird behavior of action

[matentzn]

Hey, sorry to ask here, I have been using mkdocs-deploy-gh-pages for a number of pages successfully, but for some reason, I cant get it to behave here: https://github.com/INCATools/ontology-development-kit/blob/master/.github/workflows/docs.yml

It fails very hard there:
https://github.com/INCATools/ontology-development-kit/actions/runs/1536704620

It appeared to me that somehow the requirements.txt in that repo is not ignore, hence the stupid plan of mine to add REQUIREMENTS: NONE, but that didn't change in any case the behaviour. Any idea how I can debug this? Thanks for the help.