"Error invoking the protocol buffer compiler" / "Field number 1 has already been used" after editing a document (Solved)

I was losing hair on continually getting this error message (see screenshot).

Could not save com.wm.util.coder.ProtoBufException: [ISC.0076.9021] ... Error invoking the protocol buffer compiler. Error: proto: ... Field number 1 has already been used in ...

This would occur when I tried to modify a publishable document - I just could not save it. To my horror, I could not reverse my changes either - same error!

What eventually fixed it for me was deleting the document on IS, then deleting from UM using Enterprise Manager, then (importantly). reloading the package. Only after the reload could I recreate the document successfully. IS would let me save earlier. .

This was on webMethods Integration Server 10.1 Core fix 11. Searching for this error didn’t return much, so I’m logging this in here in case it helps someone