Service Now API Invalid Character between object key and value

Just to verify it's definitely the "We found an invalid character between object key and value in the JSON input."?
Are you running table API or a custom Scripted REST API?

With API Explorer it was definitely a valid JSON payload?
Once I had validated it was a valid JSON payload I did play around with ensuring data was trimmed, but with the specific message we're getting here, none of that would have resolved the issue. There was no invalid characters, just a non-printable ASCII 20 space between name-value pair. 
I suppose there could be some quotation marks in your value that isn't properly escaped by ServiceNow, but seems odd. If you're running a Scripted REST API then start limiting the number of records returned, and make sure they have a specific order, to make your troubleshooting easier. Start with only returning a single specific record.

I had loaded BI fresh on another computer, but the gentleman that originally reported the issue really did just upgrade his BI in place.