eConnect error: Unable to update the existing SOP Header record

I was receiving said error from my Dex enhancement which uses eConnect to update SOP transaction data from my custom window.  The above error, along with the 2218 error code indicates ABSOLUTELY NOTHING!

Here is the trick to solving this.  Change the transaction from an update to an insert by way of the UpdateExisting flag.

My original transaction was an Update and eConnect simply replies "No".  Changing this to an Insert yielded a message to the fact that the salesperson was inactivated.  Activating the salesperson allowed the update to proceed just fine.

Interestingly this only stopped the SOP update via eConnect.  When changing the data inside SOP Transaction Entry it saved just fine.

Why does an inactive salesperson stop an eConnect update of SOP but not an update via GP?

*** Update 1/29/2020

We found another situation where this can happen.  In our new case the above fix DID NOT show us what the problem was.  It was only by changing various eConnect inputs did I find the cause.

Apparently there was a problem with Sales Tax on the order.  I'm still trying to home in on the exact problem but here's the gist.

There is a flag in eConnect that is CREATETAXES.  My enhancement sets this value to 0 unless the user changes a value related to sales tax.  In this case no sales tax amounts were being updated, so the value passed was remaining 0.  However for some reason (to be determined) GP REQUIRED this flag to be set for this order.  When I manually overrode this flag the error ceased and the sales tax on my order was adjusted up by $.02.

In contact with my order entry person I found she had manually updated the sales tax via the Sales Transaction Entry window to override the sales tax amount (reducing it by $.02) prior to getting this error.

Regardless after my manual override we could not attempt to recreate the error so it remains a mystery, but at least we have a solution.

I will be working to try to identify exactly what caused this error but realized in this instance that this error might occur for reasons that can't easily be discerned and only hard work and elbow grease will track down the cause.