Auto top-ups failed on Oyster - legal to ignore them?

I have read exactly what you said.

You say that you received an email about payment failing.

That means you are effectively in debit, which they have notified you.

But you are asking people here to agree that you don't owe this, and so far nobody has done so.

You can't remember the password and can't or won't get yourself a reminder, so apparently you have no way of checking the payment details or updating things when the bank issues a replacement debit or credit card with a new expiry date.

In the end, whatever the actual problem, nothing stops you owing for the travel you've so far undertaken and effectively not yet paid.