Cannot connect to either iPad or iPhone

Greetings:

Connecting my iPad Pro or iPhone 7 (both running the latest software updates, respectively iOS 12.1.1 and iOS 12.1.2) by lightening-to-USB cable to my MacBook Pro (2017 retina, running 10.14.2 Mojave) I get a message that a software update is required:

Of course I click on install, and the system reports that it is attempting to download 1 file. Invariably, however, I receive a message after about 10 seconds that the process failed:

In the mean time, the iPad or iPhone are not able to connect to iTunes (also up to date). When the following message appears, prompting me to allow access to the iPad (or iPhone):

I can hit "continue", only to see the message disappear and quickly reappear. The iPad and iPhone never show a message requesting authorization of the access, but produce a beep about once every second.

The only way to break the cycle is to disconnect the USB cable, after which I can quit iTunes.

This problem is similar, if not the same, as the one reported earlier in the older, unsolved, threat: https://discussions.apple.com/thread/8494433. It appeared very suddenly today (Jan 22, 2019), I was able to synchronize my iPhone about just one week ago without problems.

Anybody a solution?

Thanks a bunch in advance! Erik