iTunes could not connect to the iPod touch because an invalid response was received from the device.

Received the message "iTunes could not connect to the iPod touch because an invalid response was received from the device". iPod 2 works great for a music source, so would like to add music to it as long as it has capacity. My iMac and iTunes system versions are OS 10.10.5 and iTunes 12.3. iPhone 6 and iPad 2 sync and work perfectly and are recognized with either of two Apple USB cables in any of the iMac USB ports. Have restarted the iPod and iMac, uplugged and replugged, but same message appears. The iPod was last connected (successfully) and synced in Dec 2014. Only changes I am aware of since then were OS and ITunes updates and iCloud storage of music and photos. I am a pedestrian user...thanks for any help.