It looks like the sleeve to me too.

Quite common problem. You will need to drop the pan to unlock the sleeve from the inside, to replace it with a new one.
So, you will need 6L of transmission fluid, a pan gasket and filter (if metal pan) or a new pan (if plastic).

I did mine while changing my fluid and it is quite simple.

There is a procedure on this site:
ZF 6HP26 Sleeve Replacement by wilson009 pdf free ebook download from bmw.tech.officelive.com

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