A common cause of the '601' error actually has nothing to do with your bank account and is actually the browser you're using to access the website - specifically it's blocking a security script from running in the background that checks your card details are legitimate and that you're not using hacks or something.

Commonly, this occurs with Firefox with the Noscript and Adblock addons installed, which are designed to stop scripts running on webpages, unfortunately they often can't tell between a legitimate one and malware and just block it regardless.

If you're using this browser with addblocking addons/software, disable them and try the page again (this error code in fact happened to me the very first time this system went live back in 1.0 when I tried to enter my Visa debit card details - but the above cause and solution was listed here on this forum and I tried it and it worked. After 'whitelisting' the SE Account Management System/Mogstation in Noscript and Adblock Plus, I've never had this error occur since.)