Help debugging "An unknown error occurred."
https://github.com/craftcms/cms/tree/master-v2/src/services/UserSessionService.php#L325

The error is thrown when you don't have the permission to change/do something. Your file permissions do not matter it is about the permissions in craft. Are you not admin? Could you tell us if you are a normal user and don't have certain permissions? Maybe another member of your team changed something? 

Do you have any plugins installed that have any hooks/events? 
Do you have the latest Craft version?