OpenProject pg_restore - input file does not appear to be a valid archive
I'll post an answer because it might be useful to someone else.
As the screenshot in the question displays, there is an error in the /home/openproject/.postgresqlrc file on the server on which I've run the backup.
The format of that file was not correct: I've fixed it by following the format described here and when I had run the next backup I get no errors, and I was also able to restore that backup successfully.