On macOS 10.13.6, in my fork of a project, I created a new branch from master on the command line, made some changes to the code, committed them using GitHub Desktop 2.5.5, and then tried to publish the branch using GitHub Desktop. It said:
The push was rejected by the server for containing a modification to the workflow file .github/workflows/main.yml
The branch did not modify anything in the .github directory.
I was able to publish the branch from the command line.
I have published hundreds of branches to this repository and others using GitHub Desktop over the past 4 years and have never seen this error before.
I reported the problem to the developers of GitHub Desktop and they said the error message is coming from GitHub and there is nothing they can do about it and suggested I report the bug to GitHub.
Since GitHub no longer offers a way to report bugs, I am reduced to writing this post to this community. Hopefully a GitHub developer sees it and adds it to whatever internal bug tracking system they use.