Hey JK,
Thanks for your quick responses - unfortunately I am not able to keep up 
So first I tried to POST directly to the Odata service exposed from IGW, but I do not seem to get the X-CSRF-TOKEN back from a GET operation so am unable to test that out (this has happened before, it returns most of the time but sometimes its very stubborn) - so I'll try again tomorrow.
if you notice the metadata in my first post - the `updatedBy` field can be null. This usually translates to a 'null' value in the INSERT statement.
Now, regarding the original query - I tracked the record I was trying to insert with the backend SQL server. It seems the record is getting inserted after all! It was all there, just as I sent it from PostMan.
But from an offline OData perspective, the error would still mean a failure for the device making the flush call. I just might report this to SAP via a OSS message.