Cannot obtain lets encrypt certificate

I’m using FreedomBox19.21, set up DynDNS so that I can reach it by freedombox.domain.de.

I tried to remove and recreate a certificate but get the same error every time:

I notice that the error message says something about freedombox.freedombox.domain.de which seems odd.

My hostname is “freedombox” and my domain name is “freedombox.domain.de”.

What can be done to make certificates work again?