SR-52 not writing cards

[Kees Bouw]

My Texas Instruments SR-52 can't write cards, as it turns out. I repaired it a few years ago and never tested this before. I got it from eBay and it is in beautiful condition, almost new. Very little used I guess. There was a segment missing in the display so I replaced the led stick with one from a donor machine. I repaired the card reader which went very well. It reads cards without any problem. But when writing a card, the motor stops when it is halfway in and the display shows a flashing zero. Of course I covered the write protect window like it says in the manual and I checked photo's on the internet to make sure I did it correctly. I also tried some variations where I covered more or less of the surroundings. None of it makes any difference, the calculator behaves as if I inserted a write protected card. I do not know how the write protect mechanism works exactly. There is an optical component consisting of an LED and a sensor transistor. I think it has the number SDA313 on it. Which (component) failure could cause it to behave like the write protect window was not covered?
One other thing, probably unrelated to this issue: I found a loose part in the calculator near the display and I can't figure out where it belongs. See the attached pictures.
Any help is greatly appreciated!