Okay I was able to figure out the source of this issue. The problem seems to be conflict with version of IronPython that is shipped with Dynamo (2.7.3)
Rhino is shipped with 2.7.9 and the IronPython libraries belong to this version. So they natually cause an error when being loaded into an older IronPython version.
As a workaround you can disable Dynamo and RIR will work properly.
- See this https://www.rhino3d.com/inside/revit/beta/reference/toubleshooting#search-for-conflicting-plugins
- OR remove (save a backup!) the
Dynamo.addininside the%PROGRAMDATA%/Autodesk/Revit/Addins/20XXfolder