Hi Johan,
I am guessing that you may have figured this out since, when I tried to follow this email in my inbox to the copy of this message in the public forum, I found it deleted. However, I thought I would reply anyway and create a thread, to share what I found when taking a look:
I tried generating an API key in the public demo site. As a first note (in case you forgot), the option to generate a REST API key will ONLY be available on a user's edit page if you have first enabled the arRestApiPlugin via Admin > Plugins.
I did find the option to generate the key after doing so... however: I noted that in the public demo's Bootstrap 5 theme, once generated the key was in fact incorrectly labeled as an OAI-PMH API key. These are not the same keys, and should have different labels. Consequently, based on your report I have filed the following bug ticket:
Cheers,