Is it not possible to give access like this IF the users don't have EDIT permission to the contact record the button is on?
7 respuestas
Stefanie,
What happens if you do give these community users edit permissions? Does the error go away?
INSUFFICIENT_ACCESS is generally a permissions issue steming from a CRUD related permission or sometimes a profile permission is needed.
This post - https://developer.salesforce.com/forums/?id=906F000000095q0IA - Suggests that the Force.com Site user needs access to the class as well.
Cheers,
James