As I cannot think about playing without this mod I did a quick fix. Did not notice any problems with KSP 1.3.1 afterwards.
Problem was that there is a version check that sets some variables which fails with the new version.
Modified dll for 1.3.1 (copy into GameData\EditorExtensionsRedux\Plugins of the 1.3.0 version): EDIT: took link down
What I did so somebody with a better reputation can verify my change as this is a new forum account:
Modified the check in EditorExtensionsRedux.Constants
From: if (Versioning.version_major == 1 && Versioning.version_minor == 3 && Versioning.Revision == 0)
To: if (Versioning.version_major == 1 && Versioning.version_minor == 3 && Versioning.Revision == 1)