it looks like we have hit some kind of limit for item roles with this plugin. I wanted to add 4 new item roles under "Assign roles" and got the error from the screenshot.
I was able to add 3 item roles seperately, but when adding the fourth one I get the same error again. If I delete one existing item role, I am able to add a different one instead. But after that I can't add anymore item roles without getting an error message.
We have 238 item roles at the moment.
I updated the plugin to its current release (2.8.1) but unfortunately it didn't help.