Quite a common problem, the transmission has detected a fault and shifted to 'Park' to prevent damage to itself. It is likely a low oil pressure issue caused by low fluid level or a cracked bridge seal between the mechatronics valve body and the oil pump inside the main body of the transmission.
If you can get hold of INPA read the error codes from the EGS module and post them up, I would suspect you have 'Gear check errors' where the transmission is slipping.
I had this issue, sometimes I could put the car into 'R' and hit the gas and it would go forwards! Issue was the low oil pressure not being able to apply enough pressure to the clutch packs.
The bridge seal design has been revised by ZF to make it more robust.