Hello, Alex.
Thank you for visiting Apple Support Communities.
Here are the steps that I would recommend going through when receiving error 306.
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:
- In iTunes, choose iTunes > Preferences (Mac) or Edit > Preferences (PC).
- Click the Advanced tab.
- Click the "Reset cache" button.
- Click OK and see if the issue is resolved.
Adjust Parental Controls in iTunes:
- Open iTunes.
- Access iTunes preferences:
- On a Mac: From the iTunes menu, choose Preferences.
- On a Windows PC: From the Edit menu, choose Preferences.
- Click the Parental Controls tab.
- 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.
iTunes: Advanced iTunes Store troubleshooting
https://support.apple.com/kb/TS3297
Cheers,
Jason H.