HT201413: Resolve issues between iTunes and third-party security software
Learn about Resolve issues between iTunes and third-party security software
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Q: itune appear "error occur (21)" when i restore off line update from ios 7.0.4 to 8.1.2
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Jan 26, 2015 6:24 AMby Allen A,Hi Myo Ko Ko Aung,
When iTunes reports this error during the restoration of an iOS device, it typically indicates a conflict with security software.
Related errors: 2, 4, 6, 9, 1000, 1611, 9006. Sometimes security software can prevent your device from communicating with either the Apple update server or with your device.
Check your security software and settings to make sure that they aren't preventing a connection to the Apple servers.
Resolve iOS update and restore errors
http://support.apple.com/en-us/TS3694Sincerely,
Allen
