Authorizing a computer in iTunes

I find it easier to move around apps on the screen of my iPhone 6 Plus using iTunes.

Today, when I tried to do just that, I kept getting this error message:

The iPhone "my name iPhone 6 Plus" could not be synced because this computer is no longer authorized for purchased items that are on this iPhone.

As it instructs, I go to Store to "Authorize This Computer" & it says that it is already authorized - which I knew because I authorized this computer when I purchased it in January 2015. I am running Windows Pro 8.1

Using Google, I found the Apple Support document entitled "If iTunes repeatedly prompts you to authorize your computer".

I performed each separate step in the order listed.

After each step, I tried again in iTunes to move around my apps & apply the changes, but repeatedly got the same outcome.

I did update iTunes to the latest version just before I tried all of the above.

And, I also updated my iPhone 6 Plus to ios 8.3 just yesterday.

I know it must be something simple that I am missing, but just can't seem to figure what it is.