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I found the following relevant information relating to error 306 in iTunes, please see the steps included for Mac:
iTunes Store loads partially or returns "Error 306" or "Error 10054"
Proxies, parental control settings and software, security or filtering software, or a bad iTunes Store cache can cause this.
To address proxies, Remove Internet Options proxy settings and connect to the Internet without a proxy.
To reset iTunes Store cache:
1. In iTunes, choose iTunes > Preferences (Mac) or Edit > Preferences (PC).
2. Click the Advanced tab.
3. Click the "Reset cache" button.
4. Click OK and see if the issue is resolved.
Adjust Parental Controls in iTunes:
1. Open iTunes.
2. Access iTunes preferences:
1. On a Mac: From the iTunes menu, choose Preferences.
2. On a Windows PC: From the Edit menu, choose Preferences.
3. Click the Parental Controls tab.
4. Remove restrictions on Parental Controls.
For more information on parental controls or content filtering software, see iTunes 10.5 for Windows: May see performance issues and blank iTunes Store.
For more information on other security software, see iTunes: Troubleshooting security software issues.
For additional information and error messages, see the following resource: