So I'm using this HTML to RTF file converter on some of my old writing and everywhere there's a punctuation mark the converter substitutes the word "SONDERZEICHEN". What's up with that?

Some kind of strange German plot to take control of the world through subversion of our punctuation. Next thing you know all our periods will turn into Zusammentreffen and the world will go to hell in a handbasket.

Or maybe it just means special character in German.

Germans don't use punctuation. The history of their language is one long sentence.