Q: Cannot restore backup from iPhone 4S 8.1.2 to new iPhone 6s 9.2.1

Hello,

It seems that I cannot transfer my iPhone data from an old 4S to a new 6S I just purchased. After backing up data from the old iPhone and attempting to restore it on the new model, I get the following error, after which the iPhone 6S restarts:

"iTunes could not restore the iPhone because the backup was corrupt or not compatible with the iPhone that is being restored."

Key facts:

iPhone 4S is running the 8.2.1 IOS;

iPhone 6S came with a preinstalled 9.2, which I subsequently updated (after a number of failed attempts to move data) to 9.2.1;

iTunes was updated to the latest 64bit 12.3.2.35 version (also updated after the first data transfer attempts failed)

I created multiple restore files with iTunes, deleting old ones - even tried with a different USB cable in case the previous one was damaged;

The operating system is Windows 10 64bit

The one thing I haven't tried is to update the iPhone 4S to 9.2.1, create backup and then restore iPhone 6S from it, but if it's a software glitch I'm worried I'll  lose data in the process and I won't be able to restore it from the existing backup.

I've seen somewhere comments about a programme that potentially bypasses this issue but I feel somewhat reluctant to pay $30 to get the device I already paid for to work.

Has anybody experienced anything similar? Are there any working solutions available?