HTTP-01 challege: Some challenges have failed

Hi,

Yesterday I installed BW v1.6.17. Switching from Nginx reverse proxy to BW.
Installation went fine. All default values. Multisite. No e-mail address for LE. As I understand, this is not mandatory anymore.

When adding a service for a self-hosted webserver, the LE HTTP-01 challenge fails (Some challenges have failed) for a new certificate.

When switching back to Nginx (on another VM) the HTTP-01 challenge is fine.
So port forwarding is fine. So the public DNS A record is fine (pointing to my public IP address).

How can I diagnose BW? The log file isn’t very helpful (“Some challenges have failed.”).