As for some users experienced different error when restoring to new ios Wait for new update
Apple Releases New Build of iOS 9.3 for iPad 2 to Fix Activation Issues
Apple has released a new build of iOS 9.3 for the iPad 2 that likely fixes activation issues encountered by some users when updating.
When updating to iOS 9.3 on older devices, users were asked to enter their Apple ID and password. However, if the user didn't remember their password the device became stuck in an inactivated state.
"Updating some iOS devices (iPhone 5s and earlier and iPad Air and earlier) to iOS 9.3 can require entering the Apple ID and password used to set up the device in order to complete the software update," an Apple spokesperson told iMore. "In some cases, if customers do not recall their password, their device will remain in an inactivated state until they can recover or reset their password. For these older devices, we have temporarily pulled back the update and will release an updated version of iOS 9.3 in the next few days that does not require this step."
The new build of iOS 9.3 for iPad 2 is number 13E236.
i Don't have links yet but will update news section once we have it
i don't know exactly why or what happens but i personally experienced many errors with 9.3 and 9.2.1 and waiting for the new build