Hard to say anything without knowing your settings. semi-random questions:
Could it be the browser or the OS?
Could it be a https vs. http problem?
Could it be www. prefix missing or included in DNS name?
Could it be that two different DNS servers are queried when logged in vs. logged out?
If you want more chances to find the problem, post your configuration (masked/anonymized, but not “removed”, i.e. if your real DNS name is my_nice_domain.com, do not replace it with [REDACTED} but change it to another_name_domain.com), instructions here:
http://forum.mikrotik.com/t/forum-rules/173010/1