I think that plug-in might have been installed by InCopy. Did you once install it or a trial copy, and then throw it away?
Look inside your InDesign folder in the Applications folder. There should be a folder of Plug-ins, and you might find it there. You could remove it if you find it.
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Assignment UI is used by InCopy, but ships with InDesign. The error here is that the plugin is missing, not that it is causing an error.
Doug, did you maybe install an InCopy workflow solution that removes the Assignment UI plugin, and then uninstall that workflow solution? In our environment, with Woodwing SmartConnection, Assignment UI gets moved from Plug-Ins into "Plug-Outs." If that's the case for you, just move it back to Plug-Ins.
But you could also just reinstall InDesign, that would fix it.
I've found the UsernameUI and AssignmentUI in the InCopy workflow folder but I cant find any plug-outs folder only plug-ins, any ideas?
Whoops. I'm sorry, I had misread your error. I thought InDesign was looking for the Assignment UI plugin, but that's not the case.
There is no Username plugin, but there is a Username UI plugin.
This is strange.
Do you have any 3rd party extensions installed?
Does it work with a different user account?
If not, it's probably time to reinstall InDesign.