Try changing the password for those users.
I’ve had a similar experience, last week, and I tracked it down to the password.
You can check it by going to the login page and entering the password in WITHOUT a User ID. If it’s the same issue that I was getting then it’ll fail in the manner you’ve described already.
If you try that again with a different value in the password field, but leaving the User ID blank, you should get the WordPress login screen with a ‘user id / password is wrong’ type message. Then change the password for the users in question and try and log in and, hopefully, it’ll work OK.
It’s happening, across the board, on s3RiverThames server (my server). I know because I checked some other WordPress sites, on that server, that are nothing to do with me and they all suffer the same fate.
Presumably it is happening on all other Site5 servers as well? It doesn’t, however, happen on other, non-site5, servers (I checked that too – because, originally, I thought I’d discovered a really bizarre bug in WordPress).
It seems to have come on around the time that Site5 mitigated the recent brute force attacks on WordPress.
FWIW, it took me a couple of days to figure this out, plenty of swearing and lots of strong coffee as there’s no help in the error logs at all (you just get a load of 403’s and that’s about it) and trying to search for this kind of error is not easy.
@Site5 – I’d be interested to know, if you figure it out, what’s causing this. Feed the geek in me 🙂
HTH.