I’ve been informed that this has been resolved, pending release. It’s likely to be next week before this does go live (although we’ll let you know if it’s any different).
It’s worth noting that setting up a direct debit works as expected, as does collecting a payment against an invoice. I would recommend exploring this option for the moment.
I apologise for the delay, we have recently migrated over to the new GoCardless API. This had caused some issues for one-off bills that we have now resolved.
Your client should no longer be seeing this error but please let me know if you experience any further difficulty.