I was getting 601s as well, but I figured out that it was the NoScript plugin ******** it up, temporarily turned it off and it went through without any problems... So if you use it or a similar plugin try disabling it and see if it helps.
This, I'm glad I read the whole thread before I said what this guy said. I had to open all scripts globally thru NoScript in order for SE to finally accept a card. Not just any card, I tried 3 different ones before they finally accepted my debit
card. (So if my checking acct is cleaned out someday, I'll know who to blame. :P )