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.
with a minor 600+k