OK more information, I have found the latest version of Firefox in Ubuntu apparently can't login at all. IE8 has no problems. Safari on Mac or PC logs in once, then can't login again...
On attempting to login from Safari, we see this in the cpanel login_log
82.x.x.x -
[email protected] [12/22/2010:09:35:32 -0000] "GET / HTTP/1.1" FAILED LOGIN webmaild: user password hash is missing from system (user probably does not exist)
82.x.x.x -
[email protected] [12/22/2010:09:35:40 -0000] "POST /login/ HTTP/1.1" FAILED LOGIN webmaild: user password hash is missing from system (user probably does not exist)
and this in the access_log
82.x.x.x - - [12/22/2010:09:35:26 -0000] "GET / HTTP/1.1" 401 0 "" "Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10_6_5; en-us) AppleWebKit/533.19.4 (KHTML, like Gecko) Version/5.0.3 Safari/533.19.4"
82.x.x.x -
[email protected] [12/22/2010:09:35:32 -0000] "GET / HTTP/1.1" 401 0 "" "Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10_6_5; en-us) AppleWebKit/533.19.4 (KHTML, like Gecko) Version/5.0.3 Safari/533.19.4"
82.x.x.x - - [12/22/2010:09:35:32 -0000] "GET /cPanel_magic_revision_1261021795/unprotected/cpanel/style_optimized.css HTTP/1.1" 200 0 "https://server.co.uk:2096/" "Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10_6_5; en-us) AppleWebKit/533.19.4 (KHTML, like Gecko) Version/5.0.3 Safari/533.19.4"
82.x.x.x - [12/22/2010:09:35:33 -0000] "GET /cPanel_magic_revision_1261021794/unprotected/cpanel/images/mid.jpg HTTP/1.1" 200 0 "https://server.co.uk:2096/" "Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10_6_5; en-us) AppleWebKit/533.19.4 (KHTML, like Gecko) Version/5.0.3 Safari/533.19.4"
82.x.x.x - - [12/22/2010:09:35:33 -0000] "GET /cPanel_magic_revision_1261021794/unprotected/cpanel/images/webmail_login_sprite.jpg HTTP/1.1" 200 0 "https://server.co.uk:2096/" "Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10_6_5; en-us) AppleWebKit/533.19.4 (KHTML, like Gecko) Version/5.0.3 Safari/533.19.4"
82.x.x.x - - [12/22/2010:09:35:33 -0000] "GET /cPanel_magic_revision_1202352189/unprotected/cpanel/images/button-bg.jpg HTTP/1.1" 200 0 "https://server.co.uk:2096/" "Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10_6_5; en-us) AppleWebKit/533.19.4 (KHTML, like Gecko) Version/5.0.3 Safari/533.19.4"
Help!