Wife went out to start GLK250 this morning and received error message "BluTec 10 starts left." Had the same problem 6 weeks ago and dealer could not discern why (DEF tank was full), but reset codes with scan tool. Dealer topped off BluTec fluid and have only driven about 600 miles since then.

 

Strictly a guess on my part, but it sounds like a faulty level sensor in the DEF tank. (I'm assuming the DEF tank has a level sensor of some type).

 

There are 3 level sensors in the tank. Your dealer needs to properly diagnose and fix the car, "resetting" it and giving it back to you, isn't gonna fix it.