Payment Processor stops working with new checking account

Even though I have dealt with many merchant account setups and ecommerce sites within the last 9 years, for some reason Payment Processors setup in NetSuite has been the most difficult and time-consuming part of all.

Once we realized setting up multiple payment processors is buggy and unusable in NetSuite, we made all but one inactive about two years ago. Last week, I had to update the deposit account setting for the active payment processing. I simply changed the account (Setup > Accounting > Credit Card Processing) and saved the settings (re-entered the passwords too, of course). Since then, we cannot charge anything. We see the infamous error message that simply says "I don't know what the problem is, it could be the card or the settings"