" I assume that the no. 64 is to tight to 54 and thats the reason why there is basically no gap between 55 and 64 and therefore the gear 48 is blocked"

The photos and video of the loose nut make the situation much clearer now. No. 64 should be tightened firmly against 54.

The problem started when the nut became loose, allowing 54 to spin with the gear and wear against the thrust washer (64). Everything being done now is essentially compensating for the wear on the inner ring (54).

If 54 is loose, it will also vibrate, which is most likely what is causing it to jump out of gear.

If you check the length of part 54, you will find that it is shorter than 48. That is the root of the problem. As a result, thrust washer 64 is clamping hub 48 instead of the inner bearing bush 54. The previous owner may have encountered this issue and attempted to compensate by loosening the nut😪

To resolve this properly, you will need to shim 54 until the specified end-play tolerance for 48 is achieved.

You can determine the required shim thickness by measuring with Plastigage and then adding the specified end play to determine the correct shim thickness.

Once you understand how the parts interact, this becomes a straightforward fix.

Selector detent springs can also cause this problem. We discussed this in the site PM.

The problem with 54 needs to be solved before you move forward. It will only get worse and could damage the shaft beyong repair.