The #Matrix federation tester gives me an error, can someone tell if that is really a fault on my server side or if the federation tester has a fault?

See here: https://federationtester.matrix.org/#utzer.de

My #Synapse server is running on matrix.utzer.de and the users are in the form user@utzer.de, so I think a certificate valid for matrix.utzer.de is fine.

#Yunohost