The issue is that you haven’t actually uninstalled the Pro version completely. Both the Pro version and the free version share some dependencies: draw.io and mxgraph-client. The Pro version requires some newer version of these dependencies. When you uninstalled the Pro version these dependencies have remained installed. The problem is that the code of the free version hasn’t been updated to work with the newer version of draw.io and mxgraph-client, so it fails.
To fix this you need to downgrade or uninstall the newer versions of draw.io and mxgraph-client, and then re-install the free version of the Diagram application.