I had an error like this in my system, but with the CD module. After we checked it inside, then we found out, that only a cable was badly connected - after it was "pressed" to it's correct place, there was no more "error message" on the screen, and the CD player just playes all CD fine.
So, therefore I think, Your system is not able to perfectly handle the cassette mechanism - and to avoid from a bigger problem it shuts itself down. It could be a very simple problem from a not perfectly installed cable (please notice the CD mechanism replace!) up to a damaged cassette mechanism.
IMHO, if the system doesn't release the tape after a short power down, I suggest You not to try to solve the problem by Yourself, but to ask for a help from an authorised dealer, and if it's possible, the servician who replaced the CD in Your BL2500.