Barracuda NG Firewall has a comprehensive anti-malware and anti-virus scanning engine. Unfortunately, this clashes with the Apple Appstore: downloading apps and updates get blocked by the AV scanning engine. In order to resolve this, the URLs from Apple need to be excluded.
Using NGAdmin, log on to the firewall (or Control Center), navigate to the Virus-Scanner service of the firewall, and open the Virus Scanner Settings configuration node.
Click on Content Scanning and under the HTTP Multimedia Streaming section, add a new Scan Exception.
Enter a name for the exclusion, then enter these values:
Setting the allowed MIME-type to * is required as the scanning engine cannot determine the MIME-type properly, which is why the Appstore breaks in the first place. By limiting the domain to that of Apple, the exception will only apply to Apple's URLs, keeping things relatively secure.
Click OK to save, then send and activate the changes. They will take effect immediately after the change has been pushed to the firewall.