Next time try shutting the car off and then see if it will go to park. The problem I envision would not be the case if you are truely able to get to reverse.
That condition is the sticking of the governor. At 10kph the first piston of the governor opens the fluid to the control sector. The pressure instantly goes from zero to a few pounds. At this time the reverse interlock is hydraulically engaged and its not possible to shift to reverse.
If the piston sticks this pressure continues at a stop causing the shifter to not go past reverse till the car is shut-off.
If this isn't the case I would look for external linkage problems.
Bosch Master, ASE Master, L1
33 years MB technician