Had to go over to firefox... It is a real pain in the rear! I need the net for work. Loosing lots of time today, Thanks Apple!
As to the question raised by the OP about not being able to login on certain websites I have experienced the problem as well even though I am running Leopard, not Snow Leopard.
I believe I may have found one cause though. After going through the clearing of cache and cookies and so on I wound up uninstalling all of the extensions I had installed in Safari and was able to log into the most troublesome website. I have not had an opportunity to try some of the https websites I have experienced trouble with, all of which I was able to log into with Firefox. I do note that at least one of the companies was aware of some undetermined problems with Safari based on customer input.
Anyway, I had turned off the extensions, but did not realize that every time I was restarting Safari after clearing cache and cookies for the problem website that they turned themselves back on. So I gave up and uninstalled them all. I also note that Safari extensions do not have a "disable" button the way that Firefox add-ons do which can be used for testing individual add-ons to identify the problem one...Apple really should have this in the extensions manager.
Now I get to go through the process of adding Safari extensions back one at a time and testing to see if things still work.
The bottom line is try disabling or, if necessary, uninstalling them, then clearing the cache and cookies for the problem websites and see if it helps.
There is always the prospect of a corrupt plist file as well.
