"We’re sorry, but an error with our renewal system means you’ve been charged the wrong rates since 9 August 2018 - and have underpaid for your energy by £0.00. To fix this, we’ll write off the money you owe us because this glitch is our fault. You’ll need to choose a new plan, because your contract has finished"
Best answer by Transparent
I have no idea why either of you should be choosing a new contract. "Contract means Contract". I don't see how one party can make an administrative mess of it, and then require the other side to take action to mitigate this. Still less can one side terminate a contract unilaterally.
Maybe one contract end date is different to another, for gas and electricity. See the contract details in MyOVO
All Contracts are covered by the Consumer Rights Act, which you are still covered by... provided that you don't ditch it and take out another one, of course! I can expand on this if you want me to.
I suggest you handle this via email to firstname.lastname@example.org. That way you retain written evidence in case this escalates to the stage of a complaint.