iTunes 12: synchronization failure

Here was my configuration 6 months ago:

MacBook Pro ——USB cable—→ iPhone

MacOS X 10.10.5 iOS 9.3.5

I am regularly synchronizing my Mac calendars and address books toward my iPhone

(this way only) with a direct connection with my USB cable. (I don't use iCloud).

This was regularly working without any problem with iTunes 12.5.

And without any evidence of any relation, one day, this synchronization mechanism stopped working for my calendars.

I only noticed this error many days later, because iTunes didn't display any error message, and

because the missing event on my iPhone was for the next week.

I fully described the problem to Apple Support and they told me that this synchronization failure

"might come" from the fact that my iOS version was too old.

To follow the Apple procedure I upgraded my iPhone to iOS 11.1.

So now my configuration is:

MacBook Pro ——USB cable—→ iPhone

MacOS X 10.10.5 iOS 11.1

And my synchronization problem disappeared for 6 full months.

But a few days ago I set a meeting for the next week on my Mac, and as usually I synchronized

it on my iPhone. Today I noticed that this event is not present on my iPhone.

(And in case I could have made a stupid error, I repeated a synchronization process.

Same failure.)

Moreover I noticed a specific error message appearing in /var/log/system.log :

Dec 14 19:59:06 ••My_MacBook Pro••.local AppleMobileSync[90323]: [0x7fb4c1d1c280] |ISyncManager|Warning| Could not retrieve ISyncClient with id com.apple.Keychain!

What may be the origin of this lost synchronization common to 2 different versions of iTunes (12.5 and now 12.6) and two versions of iOS?

Is it normal that iTunes doesn't produce any error message when such a synchronization is

failing?

Is it possible to turn iTunes in a debugging mode where it would give an error message?

Thank you in advance for any help, even with complex command lines 😎.