You'll have to excuse me, i do't use ReviewBoard myself so just relaying messages from our dev team.
When you do this then you are unable to add a diff at a later stage.
It creates the form as a non-repository form.
The workflow we sometimes use is:
1. Create form and fill in details using web browser.
2. From the server generate a diff and update the RB form
3. Reviewer add any comments they have to an email and then additionally attaches to the RB form.
We can create a non-repository form as you have seen, but when we then try and use a P4 changelist Id to generate a diff and attach to the form, it results in an error.
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Thanks
Rob