I am having the same problem.

I believe that your security token has expired. Please run the registration wizard with an admin account to refresh the token. This can be done from the Appit Admin page by clicking on "Registration Wizard"

Thanks you Jered. Why does this happen? How often the token expires?
Regards...

The token expires in one of two ways. If no activity has been present on the server within 14 days of the last time a user logged in and used Appit, the token will expire. Warnings that the token is going to expire will begin 7 days before the expiration date. These messages are sent to all members of the OAuth Notification role. Once a user logs in and interacts with any K2 Appit components, the 14-day timer is reset.

A token will also expire after 90 days of first issuance. This timer is only reset by refreshing the token (typically achieved by running the registration wizard).

More information about token expiration can be found in the following kb article:

http://help.k2.com/onlinehelp/K2Appit/UserGuide/current/default.htm#Management_and_Administration/Admin_Token_Refresh.htm#tracksearch=token expire

Perfect, then every 90 days must apply for a token, because it is a security measure that has the K2 Appit.