Hi @Julian-huebner_9077,
This error is occurring while ProGet is trying to generate the "base url". There are a few inputs that go into this:
Admin > Advanced Settings > Base URL
X-Forwarded Headers, set by a reverse proxy like ngnix
If any of those have an invalid host name (which is what the error is indicating), then you'll get this error. In most cases, it's a typo in the X-forwarded headers.
Thanks,
Alana