The app has been having issues. Different ways to pay here Guide: How to Pay Your Bill (Contract)
You can also reset you direct debit again by contacting customer services Guide: How to find help & contact O2 or by completing this form http://static.o2.co.uk/assets2/pdf/O2_DirectDebitMandate.pdf
Hope that helps and welcome to the forum
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I am assuming you've updated your address with O2 and the Bank - address checks can be part of the online payment process. As others have said the app seems to be a bit wobbly at the moment - I make card payments from MyO2 on my laptop - never had issues there. So maybe try that instead of the app if you can't think of another reason for the payment to fail. You also say you cancelled the direct debit some years ago and been using the card ever since - it's not expired has it?
Hopefully, if O2 are showing the correct postcode, the system should update at midnight and you can make payment in the morning. Check they have exactly the correct address in My O2.
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