For some reason I get that problem when logging in from work, but only occasionally from home. Not sure where the issue is, perhaps something to do with how the server is setting the cookies?
That's exactly what it is. You could try setting your security setting a little lower. It happened to me at first when the forum went down, but I cleared my cache and cookies and it hasn't done it since. It did the same thing on my new computer until I changed my security setting to medium-low. Also if you turn on autofill in windows, it'll drop your user name and password in for you when you click in the boxes.