Click on "Copy URL" returns error

Vikunja was installed as a Docker container.
Uploading of attachments works fine. I also can download uploaded attachments by clicking on “Download”, but click on “Copy URL” returns error. It works fine on “try” installation so probably not a bug, but how to debug this?

Sounds like #2289 - Copy URL of an attachment give Error - frontend - Gitea

Yep. Don’t see anything in docker log. Are there additional logs to check?

What about the browser console?

No errors in the browser console