No reverse after fluid change

[falloutboy]

Hi folks,
well I finally done my overdue transmission fluid change. And after restarting and initial check, all was fine.
Wanted to back out the garage for a short test drive, and heard some rumble and plopping.
Shifted through all gears again, and didn't hear anything anymore. Went for the test drive an all was fine and good.

Now, 2 short drives later, I tried to backup the incline of my driveway and - well - I kinda don't have reverse and the car just rolled down again.
I feel it shifting but after the little jiggle I can just rev it like 5k before it even thinks about moving.

The changed fluid was not burned or smelly, despite having some brown/blackish varnish in the pan, but nothing else that looked suspicious.

Any ideas what could've gone wrong?
Overfill? Underfill (which I doubt thanks to the check plug) Solenoid? or did I sheer something off?
Like the flanges on one of the sun-drives

Got about 250 000 km on it, and never had any issues or strange behavior of it. Shifted soft but definite for years now.

Any hints and ideas how to proceed and have it running again till after the holidays?