Trying to populate a URL Type field with an invalid URL format produces an error
A dataset will allow, although grey out, an invalid URL but DataSync produces an error message. My preference would be that if the portal, itself, allows the value, DataSync allow it. Dropping invalid values might be ok if necessary.
I am not in a position to give the specific example to reproduce the error since it is a private dataset under development (happy to provide the link to Adrian or someone else at Socrata but cannot post it quite this publicly) and I am changing the field to Plain Text in order to be able to proceed for now. However, I suspect it would be easy to reproduce with a test dataset. Leaving off the top level domain and using a comma instead of a period both create this error, although they are not the only conditions that produce it.