Hi all, I had exactly the same problem with a Beocenter 8500, it was purchased from one of the guys here on the forum and posted to the U.K.
it arrived in perfect condition and when I was home for a few months I used it every day, very pleased with it
anyway I didn't have space for it when I returned to Thailand so my friend brought it over with him on a vist over Xmas, it was well wrapped in a suitcase and arrived undamaged
however when I tested it the cassette door would only go half way and got stuck, it closed o.k. But still got stuck half way on each attempt to open it
I followed the instructions in the service manual and opened up the top to see if anything obvious was blocking the door, it looked like the string had jumped and got caught on something due to the rough handling of the suitcase
anyhow I put the top back down and it worked perfectly and has done ever since
probably the same problem you have, take the top up and have a look, not a big job if I can do it as I have no electronic skills whatsover