Hi Troy,
So we did some more testing and it turns out that this issue is in fact also occurring when the application is run directly from SAP. So this rules out that it's a portal issue. YAY!
I had my guys check out some things again and it turns out there was an errant trigger of this message that was causing the weird behaviour. The dubious code has now been removed and so far so good.
We are happy enough that we have gotten to the route of the issue and must apologise for wasting your time.
Many thanks for your help.
All the best,
Liz,