In the JIRA admin interface, if you upgrade the application Link to FishEye (because JIRA or FishEye has been upgraded for instance), then the config.xml is updated from <proxy-info/> to <proxy-info proxy-host="" proxy-scheme=""/> and this in turns causes the following situation if then trying to access the admin console of FishEye:

The requested resource cannot be found.
There is no Action mapped for namespace /admin/:/admin and action name login-default. Check if there is such an action name with such namespace defined in the xwork.xml and also if such an action class exists. Check also the log to see if the action class is successfully loaded.