You,ve found the problem
Brothe of The Benz, gehard
You're no worse for the wear after removing the sump. I can't understand how so many are having problems with drain plugs?
The "stuff" in the pan is band and clutch material after it has failed.
There's nothing else one can pass on to you except to choose your repair shop very carefully.
When mine fails I'm going to my MB dealer.
I feel they are more knowledgable about my transmission than a shop that services all brands.
Happy Trails Beep Beep from the Spiderman in Houston!!!