Payment not processed Invalid Order State

Hi, we’ve been using the plugin for a few days now and it’s been great but we have just had a customer get in touch saying they had an error ordering and so they tried again with no luck (apparently to them) however the orders from our backend look fine so we have a duplicate. Their screen stated ‘Your payment could not be processed Reason: Invalid Order State’
So the order actually comes through but the customer thinks there has been an error whilst making it.
Any reason? Cheers in advance

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