Hi Awais16,
It would seem to me that if you can login once you should be able to do it at anytime (server(s) active of course!), but here's what I'd do: delete/empty my browser(s) cache and associated temporary files, passwords, the whole nine yards (for online and offline content(s)), check my firewall settings for exclusions, re-check the first party cookies enablement(s) (you should not need third party cookies), re-check the security level(s) and associated, and when logging back in don't assume you know where you are at - but . . ., rather know where you are at by looking around the successive page(s) and be aware of your surroundings in the event there is a second login that you are overlooking.
Most errors are operator errors and can be allieviated by a sound footing. If this still fails attempt login with another browser (preferably the generic, IE6+).
If your problem still persists update this thread for assistance again, I'm fairly new to Zymic myself but I've never had a problem with logging in to any facet (other than my own ignorance (which I overcome, alot)). Hope this helps.