Someone else had a problem that was resolved in a way that might be related. In short, the dependencies (and dependency order) the tools are using to execute might not be set up correctly. Galaxy needs to use a fresh version of miniconda that it will set up itself.
Does this help? → planemo test/serve conda package installation
Other than that, this type of issue can be related to Python version problems in the full server implementation. We didn’t attract a developer yet since they might have confused this question with the other since they were so similar. We can ping them again.