HTTP 403 is a permission issue.
I'd hop on console and take a look at the permission for the files under /opt/mwg/storage/default/2014-06-03_07-35-44-468_+0200/error_templates/' (typically index.html should be in the subfolder 'default').
All files should have '644' permissions (-rw-r--r--)
I also find it odd that the 403 points to /opt/mwg/storage/default/2014-06-03_07-35-44-468_+0200/error_templates/, when in fact index.html is in a subfolder under that directory.
I solved the problem now --> It is necesarry that the language setting was configured to "force to de (german)" ..... depends on the template configuration.