Sounds like the paint protection has reacted with the window trim sealant. I've just replaced both rear sill trims (MB calls then 'garnishes') on an ML63, cost about AUS$100 each OEM. The actual R&R is quite straightforward, requires no special tools, I did it myself following this forum post:
https://www.benzworld.org/forums/w1...ement-garnish-moulding-most-people-would.html
https://www.benzworld.org/forums/w1...ement-garnish-moulding-most-people-would.html