Help debugging "An unknown error occurred." Recently, when I edit categories or entries I get the dreaded "An unknown error occurred." When I check the logs I see:

2017/12/11 20:41:51 [error] [exception.Craft\HttpException.403] Craft\HttpException in /srv/users/serverpilot/apps/0default/craft/app/services/UserSessionService.php:329


Not quite sure where to go from there. 403 makes it look like a permissions issue but my permissions are all setup just like I normally have them (744). Anyone got any ideas?
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?