Error: xxx.visual.force.com says - When opening an Event in CRM
The user does not have access to the Event record.
Be sure the user has access to the Event record.
One method is to adjust the Sharing Settings. Navigate to Setup --> Administration Setup --> Security Controls --> Sharing Settings, select Event and then adjust the Sharing Settings accordingly.
Please refer to this guide if needed: Create Sharing Rules
CRM Help Documentation: N/A