My 3350 is in the shop(again) with the OP's symptoms also. In my case, it would never give an error code and run fine at their shop(can't fix what is not broke)
It's had a history of this same issue and has really gotten bad this year. I even was gonna trade it an "L" series, but the L's are heavier duty and but not suited for me in my handicapped condition. 3350 has better amenities also.
The dealership has worked with me and asked me to continue to use it. Shop Forman and I both agreed we needed it to go into a hard failure so they had something to repair. They were also on standby to come to get it at a moments notice. I did accomplish that a week ago.
It stopped going into regen and would just light the "check engine" light. Restarting it would clear the light. After a day this it finally stayed solid with a beeping sound every 2 seconds. And finally shows Err21 (CAN communication trouble)
Called the shop Forman the code and he read to me a service bulleton that came out last month for the B3350 that describes all my past issues to a T. Seems the ECM and cabling are to be replaced. I will do a followup post after I get it back and use it .