I am no tramsission person, but I know what problems I've had in the past and so can only base on that. My Ford truck did that hesitation thing then I replaced the torque converter and it went into gear Now. But, I had also changed the filter and fluid, so it may have beena combination or one or the other. I have no idea if a MB has a filter or not.
Does the fluid smell burnt or is it a brownish color? If it is a dark color, almost black - it is rebuild time!
If it is a brownish color, you may be able to save the day by changing the fluid and filter if it has one.
If the fluid is a nice red color, I haven't a clue! Other than maybe a torque converter.