IPhone won't synchronize completely

iPhone 6 goes through several synchronization steps, but stalls on the last one. I eject the phone, try again, and an error message says "Impossible synchronize iPhone because the synchronization session was not properly started" (message translated from Italian). Apple's support pages do not tell me where to go from there. Can anyone give a suggestion? iTunes is up to date, and a companion iPad Pro synchronizes properly.

What are you trying to sync? Help us to understand what it is you're trying to do. When I sync my iPhone 7+ using iTunes it's usually to sync a new app or add a ringtone, wallpaper, etc. And yes, the iPhone will go thru several sync steps. It sounds like your phone may have some corruption - perhaps an app or something else. You may have to back it up and reset it if you can get it to sync properly.

Thank you for your quick reply, and apologies for my delayed response (due to several days of provider blackout). Tonight, after finally getting the modem back on line, I put iPhone on the rack, and it synchronized without complaint. Perhaps it ironed out its own little problem.