Hello,
I just experienced that if you create a new class for OMChroma but save and close the workspace without completing the initialisation of the slots, it could happen that next time you start OM it won’t recognise the OMChroma library, thus failing to load OM.
My solution was to delete the class in the Chroma user folder. Of course, I have to re-create it, but won’t take long since I saved the text file with the csound orchestra.