We restrict access to our blog's wp-admin/ via htaccess to trusted IP ranges.
We've found that when using a new F5 SSL VPN (which has a fancy web reverse-proxy) that login attempts fail. It's not htaccess doing this, all the requests do come from authorised IPs, but submitting the login form fails, and you're dumped back to the wordpress login screen.
Logins work absolutely fine when not across the vpn. Other web apps work fine over the ssl vpn.
Has anyone come across this issue, i appreciate this is a pretty uncommon setp - it's a fairly difficult one to debug what is going wrong. Have disabled all plugins; no difference.