You are promising a content type of application/json but are sending a plain JS Object, which gets serialised as percentile-encoded-string by jQuery. This serialization might be far from valid JSON.
Change:
data: {'partyId':party,'PartySelCombo':valueFrom,'DocumentId':DocId},
to:
data: JSON.stringify({'partyId':party,'PartySelCombo':valueFrom,'DocumentId':DocId}),