this is something which cannot be fixed at the moment. We are already discussing this on the german forum:
Bei jedem neuen Spielstart erscheint bei mir Attach-Error
It is probably caused by some trailer or attachment. So as far as we know this is only "cosmetic" so you should not see any problems because of this message. In the message there is ID 64. You can check all your vehicles (pressing and holding ALT) which one has ID 64 but it should still work as expected.
But you are right, it will display always when you load a game.
today the ID is 69 (after load my game)... lol
it's the joskin wago (the trailer to transport balls), but it's well attached to a tractor, and yes I can use it normally...
Seems to be fixed at least for me