It is normal for door retainer links to break down occasionally. Sometimes they start to warn you by a cracking sound when opening and closing the doors, sometimes they just fail to function any longer.
They need not to be maintained; they just need to be installed with a lot of grease inside the retainer link.
The newer MB models have a cap over the retainer to prevent them from water running in (and these last a little longer); the older ones didn't have that.

greetingz,

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1990 300SL-24
1993 C250D with a minor 600+k kilometer
www.MBenzNL.com(the Netherlands)