Yes, that was it!
Until now I had ProxyPass /.well-known ! which I changed to ProxyPass /.well-known/acme-challenge !
Maybe this should be added to the howto (which I’m unable to find at the moment…) for setting up self-hosted Discourse instances?
RequestHeader set X-Forwarded-Proto "https"
ProxyPreserveHost On
ProxyRequests Off
ProxyPass /.well-known/acme-challenge !
ProxyPass / http://localhost:8080/
ProxyPassReverse / http://localhost:8080/
Thank you guys for being so patient with me. 