403 error with no reason

403 forbidden means that your browser isn’t able to load the scripts up because your server isn’t allowing it. This is a hard one to solve because anything could be blocking the server:
– Most likely this is a server permissions/ownership issue, so ask your host what’s going on
– check your .htaccess files for anything strange.
– try a full Manual Upgrade, to make sure all the WordPress files are there and correct..
– Make sure to backup everything (including database) before doing any action, in case.