I had posted this wrongful in the "Product-Ideas"-area before where it stayed ignored by FMI for 2 years, but I have deleted it there and posted it here. Hoping that FMI finally notices this annoying BUG.
If custom-menus are active and there is an custom-command for "paste" then it is impossible to paste something with the keyboard shortcut for it, in the "Dataviewer" nor is it in the "Manage Database..."-dialog. After you close the particular dialog, you'll be presented with the error-dialog for as many times as you've typed ctrl-V or cmd-V.
The bug is simple to solve for FMI (which is FM12 though FM16) by simply switch of the CM when either the "Dataviewer" or the "Manage Database..."-dialog has focus. Both dialogs can only be used if a user has "[Full Access]" privileges anyway.