Hi Juan,
Seem like user which your trying to login is not part admin group. Refer below link
http://www.cisco.com/c/en/us/support/docs/voice-unified-communications/unified-communications-manager-express/110984-cme-gui-error-00.html#login
On CLI on CUE execute "show group detail groupname Administrators" command output will shows members of the Administrators group
Refer below link has procedure to reset CUE password
http://www.cisco.com/c/en/us/support/docs/voice-unified-communications/unity-express/64234-cue-gui-pw.html
HTH
Regards,
Mohammed Noor