403 errors usually mean your permissions are incorrect. The most common reason is that you've selected 777 directory permissions on a server that requires 755. You'll need to switch to 755 at Admin -> Settings -> System and manually chmod the applicable directories to 755 if this is the case.

You might also have chmoded a directory to 666. 666 makes directories unusable, directories require execution permissions. You will have to find the directory you've misassigned permissions for and change it to 755.