Assuming you're not doing something weird like having the capslock on while you're typing your password

could it be something like a cookie issue? Have you been able to try logging on with a different browser and/or a different computer?
It's _very_ weird that it's happening even with a newly created ID; I've never heard of any problem vaguely similar to that (if you're talking about yahoogroups, not other yahoo services), and I read four or five busy yahoo mod lists regularly. Sorry I can't be of more help.
Lara