Error documents: what are and how to get rid off them?

I’m new here in the community.
I have started an instance of YaCy, until now I’m quite happy, but there are yet a couple of things that I don’t fully understand (ok, maybe more than a couple).
One of them are the “error documents”. When I enter the “Index Import/Export” there is this message there:
The local index currently contains 7,742,432 documents, only 6,781,976 exportable with status code 200 - the remaining are error documents.
Did somebody have the same problem? Is there a way to fix it?