When I tried to modify the “Document schemas” (Collection) property on an XML Validation Pipeline Component, I got this error:

I don’t have the total solution, but the problem was that I had an EDI210 standard schema and an EDI210 custom schema.
When the custom schema was added as a reference at the same time as the standard one, then this error occurs on all XML Validation pipeline components in the project. Apparently some properties overlap.