Thanks for the update. Using this information, I was able to replicate the error on my end as well. It appears that if you pass a string for the Contacts field, the API runs into an error since it's expecting an integer for the RecordID. This error isn't being handled correctly however, and we're sending the wrong information to the user regarding what went wrong.
I'll update the ticket so that we can address this issue and post back once it has been resolved. In the mean time, if you come across this error again, just make sure you're passing the correct data type for the field.