Mine does this fairly frequently.
Disconnecting the power resets it and it will accept the magazine again.
MrRog removed the retaining plates from the sides of the unit so it can be pulled out easily to disconnect the power.
I followed Kevin Wood's instructions to disable the security code so I don't have to enter it after disconnecting the power.
The procedure is as follows:
1) Disable code protection on the unit (optional).
I found it convenient to disable code protection on the CCR2006 before starting work. This is because I knew the unit would be in and out of the car a couple of times during the procedure and because I wanted to test it on the bench (without the MID connected!) once modified. I suspected that entering the code without a MID would not be easy.
The code can be disabled by powering up the unit with AS held down until a beep is heard, then using buttons 1,2,3 and 4 to change each of the digits displayed until the displayed code matches that on the car pass. Finally, AS should be held down once more until a beep is heard.
Removal of the code can be verified by powering up the radio and checking that "CODE" does not appear on the MID.