CB Version: 2.4.3.
Install the latest build release and see if your issue persists.
I go into CB User Management and select either one or a number of users and then select more than one Group to assign to the users. Press the process button and get the error above. Any assistance would be greatly appreciated as I have 200 users to assign two groups too.
If you need this done now then I suggest just using Joomlas batch tool for this in Users > Manage.