If you don't use one of the keys for a long time, they can come out of synch, with the rolling code algorithim not working properly. If memory serves, there is some way to remedy this without visiting the dealer. I just don't know it!! Sorry...
If that doesn't work, you may need a new key.
And no, there is no issue with E300D keys. They are the standard MB key, not specific to the model at all...
1998 C230 "Black Betty"