The O-ring thing may be valid - can't say - not familiar with your model.

MB has max. torque values outlined in service manuals for the tranny pan bolts. They are seemingly low, but if not adhered to, can cause a similar situation. Applying too much torque causes an indentation in the pans edge. This gives the fluid an escape route.

I've been through this with one Asian SUV.

Bottom line - dont over-torque tranny pan bolts.