As an info for anyone having similar problem: try running 'python plugins_mirror.py path_to_plugin_archive.zip' from setup folder in the official calibre repo
here and debug from there. If everything is fine, it should display all required data fields from the plugin definition correctly scrapped from __init__.py
I tried it after another issue had appeared and it was the matter of adding type annotations to all variables and functions, including members of my Source class. With those in place plugin could not be parsed, removing them for the members solved the problem.