My apologies for reporting this so late.
HipChat server was updated on March 5th for a security update. This seemed to be small update that should not have any effects. As it turns out this was not the case. During this upgrade the authentication mechanism had changed, resulting in an issue where people could not authenticate. If you had a HipChat session open, it would continue to be validated, if you did not have an open HipChat session the server could not authenticate you.
Also during this upgrade XMPP was disabled (which is now enabled again).
Again apologies, if ever something does not work as expected, feel free to contact us at firstname.lastname@example.org