Note: We are working on fixing this up permanently, but we have a temporary workaround for you.
If you create a new campaign, add a Contact with the API with NO custom invite data, then you can freely upload contacts with custom invite data to the same campaign only.
You would need to do these steps with any newly created campaigns until we can get a permanent fix.
Hi all, this should be fixed now. Please let us know if you continue to see problems!