Thank you for your post.
I have been trying different scenarios to replicate this error message, but not having any luck.
Can you describe your tables and fields in greater detail? Also, what fields do you have in your portal? Do any of them have auto-enter or validation options? Do you get the error when you immediately access the field, or when you press Submit? Any information you can provide to help me replicate the issue would be appreciated.
I know it has been a long time, but was this ever resolved? I am having the exact same problem. I have a portal that I am trying to update using IWP. I have a button that basically just refreshs the page, but on IWP I get a 'Missing Join value for creating related account'. The related record is based on two fields - a serial number and type field. In the parent field type is a calc field always = 'Book' and in the child table type can change (ie. Book, Magazine etc.). That way it is displayed in the correct portal. Any help would be greatly appreciated.
I seem to recall I figured out how to fix this. I'll check back on my project and see if I can remember specifically what I did.