Is that 21.01 or 21.05 ?
You should have either an exception in the logs or a non-200 status code. If you can post that that might help
We did finally find a situation where this could happen, [21.05] Do not persist tool state when invoking workflow by mvdbeek · Pull Request #12141 · galaxyproject/galaxy · GitHub
It might help to update to the latest commit on the 21.05 branch.