I had this issue on one of our users, and the solution was to Delete BIS CICAL Service Book on BB device and resend Service Book from BES server.

To delete CICAL on BB device go to Settings–>Advanced Options –> Service Books
Locate BIS Cical and delete it.

Than go to your BlackBerry Enterprise server , right click on user and pres “Resend Service Book”

Restart your device and meeting accepts should start working.

You may also like -

BES 5 make Shift+Delete workingDefault behavior of BES server is to avoid deleting emails with shift+delete.The BES needs to 'see' the deleted emails in the deleted items folder ... BES 5.0 remote wipe Blackberry DeviceIn BES 4.1 there was an option to remotely wipe BlackBerry devices. In BES 5 it exists to , but you need to dig ... Blackberry HTTP Error 413: Request Entity Too LargeTo fix that you need to open BES manager on your blackberry server. Select servername_MDS-CS_1 Select the Connections Tab and click Edit Properties Under ... HTTP Error 503: Service Unavailable on Black Berry deviceYou may receive "HTTP Error 503: Service Unavailable" error when you are using Black Berry browser.If your BlackBerry Mobile Data System (BlackBerry MDS) service ...