Why am I receiving an ' Unable to retrieve card details from iATS. Error: Invalid Username or Password' Error?
Two thoughts: 

1) you have an iATS admin agent code 01 (and possibly more: 02, 03) and iATS agent codes 80, 81 and up for payment processing. Is it possible that the payment processor agent code/passwd that this particular contribution is linked to did have a password edit?

2) does your passwd for the agent code linked to that particular recurring series perhaps have any non-alphanumeric characters in it? Those are not recommended as we have seen them work in some cases but fail in others.