If you see this message
This happens when the login page is used and when it is already assigned to an other feature.
To re-access your dashboard, just rename "woffice" as "woffice2" in wp-content/themes/ - that will switch you back to the default Wordpress theme and without any data loss.
You can now access to your Admin, then :
You can turn Woffice back on and everything should be fine Let us know if you have any question.