Happened to me once.
The tracks are just dry.
You simply need to lubricate the tracks with an all-purpose grease.
Mercedes sells a grease specifically for this application that I saw at my mechanics, but I don't know if it is any different than just plain grease.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".