HTTP error 403 usually means permission issues. Test this on the computer that host the BlackBerry MDS Connection Service.
1/ Open IE browser
2/ Browse to the site that has the 403 error
3/ Configure the IE browser to use the proxy correctly until you can access the site
MDS Connection Service is just a medium/broker/gateway that pass the web traffic between the device and the site. To be able to browse successfully for all allowed sites on the IE that host the MDS Connection Service will be the key to your resolution for this issue.
Native but 4th class citizen of a nation governed by idiots who import congestions & contention.