Thanks for the reply, @pamtbaau
Well, good critique 
The search you point to I did. The configuration that @arank points to in this thread (“system session.enabled: true” and “system session.initialize: false”) doesn’t work for me.
The thread is 4 years old and the Grav version is 1.4, so I gather a lot of things have changed. I don’t know how to implement the solution proposed by @chris_jung. Can the admin plugin be used from a subdomain? I haven’t seen anything in the documentation or the forum.
The next matching thread is unanswered.
I have no in-depth knowledge of Grav, I am a newcomer (and very happy to have found this CMS). Maybe I should have started this thread in the support forum? If so, I’ll change it in a moment…
Any clue that can help me or a thread I couldn’t find?
Thank you very much