Oops, my apologies. It's been so long since I looked at a DNN 2 install that I forgot that the keys weren't added until DNN 3.

no, starting from 2.1.1 shouldn't matter. It also shouldn't matter which 3.x version you go too - many of the references you'll find are based on the fact that certain versions were buggier than others. If you're saying that some of the upgrades failed that may well be your problem i.e. a 3rd party module might have made changes that cause a dotnetnuke update script fail - check in your \Providers\DataProviders\SqlDataProvider folder for *.log files and see what the errors say.

What do you mean by login screen, the same type of login screen you get at dotnetnuke.com or a pop-up logging screen (which would indicate a permissions problem).