After iOS upgrade: "iPhone is synced with another iTunes library" - BUT it's not!

So, I finally upgraded my iPhone 4S to iOS 8.3 this weekend:

the overall process went smooth & I actually think the phone running somewhat faster than on the old iOS 6, so all good there!

Except that I no longer have music on my iPhone - apart for a random collection of 113 songs that show up with an iCloud icon next to them.

On closer inspection, these 113 songs seem to be previous iTunes Store purchases.

I just connected the iPhone back to iTunes, but when selecting Sync Music (which was unchecked), this warning window pops up:

"The iPhone "My iPhone Name" is synced with another iTunes library on “Visuals Notebook”

Here's the deal ... there is NO computer named "Visuals Notebook" on our network!

I have no idea why the phone would have been synched with any other library - I only use my one MacBook Pro to sync my iPhone.

And I rather not click OK to a warning that states:

"An iPhone can be synced with only one iTunes library at a time.

Erasing and syncing replaces the contents of this iPhone with the contents of this iTunes library."

Does anyone have an idea what's going on? How did this sync conflict happen?

An most importantly, how can I get my music back onto the phone without losing everything else (especially photos)?