Waiting for changes to be applied, forever

When I boot up my PC with iPhone 5s connected, iTunes starts. It syncs my iPad and iPhone 5s. iPad is synced and iPhone 5s synchronisation freezes on step 7 of 7. I get:

"Syncing "... iPhone 5s" (Step 7 of 7)
Waiting for changes to be applied".

It stays forever, unless I X the synchronisation.

If I X the synchronisation and start it anew, the synchronisation takes place without a slightest issue.

I get this situation every time I start my computer with iPhone 5s connected to it.