Situation
You have started an export via Rechnungsdatenservice 1.0. The export of one or more documents fails with the error 'Invalid document type'.
Cause
One or more documents use a document type that no longer exists in Belege online. It has probably been deleted.
Resolution
Synchronize the document types once again. You do this in 'Settings' under 'DATEV-Datenservices'. In the section 'Document types' you will find a small link 'Update this list' in the first paragraph of the section.
Now you have to edit the erroneous document(s) from the export to select another document type.
Click here for the official DATEV documentation for error #DCO01010 (in German).