Please accept our apologies for the inconvenience caused. this appears to be a bug on Sitefinity Community edition and it is logged in PITS
where you can track its progress until resolved.
For a workaround set all users the administrator role and you will avoid this error. You can edit the permissions for the administrator`s role from Administration --> Permissions.
If you need further assistance don`t hesitate to write back.
the Telerik team