I am re-posting this question because further work has shown that the original post title was not accurate, and the post had ground to a halt.
The problem is that our plugin, which is derived from a rhino 5 commons plugin, loads ok but is treated as disabled on the next rhino launch.
We normally install plugins for Rhino 4, 5 and now 6 programmatically, but the same problem occurs during a hand install, so I will show the steps for that:
a) install the plugin from the options by hand:
Plugin appears to be loaded OK:
Plugin responds to commands , all looks OK:
But on next rhino start this message box comes up:
And looking in the options plugins we see that the plugin is disabled