My Quicken Deluxe 2010 suddenly quit being able to download data from one of my FIs, using One Step Update. Trying to update the account using "Account Actions > Update Now" in the account display didn't work, either.
An error popup advised to delete the One Step Update setup for that FI, and then re-enable it. That didn't work.
I could connect to the FI and see my account data using the browser in Quicken, and using Firefox - so it was not a problem at the FI.
I used the "chat" feature on the Quicken support site to ask how this could be fixed. After some discussion they asked me to email them the OFXLOG.txt, and CONNLOG.txt files that Quicken generates. I did, and with that they were able to diagnose and fix the problem, which apparently was some kind of server connection issue between Quicken and that FI.
There are many reports of poor customer support, but this is not one of them.
"YMMV"