ITunes Sync error -54 with Iphone 6

Hi, my Iphone 6 (A1586) won't sync in Itunes 12.6.1:

Attempting to copy to the disk "Macintosh HD" failed. An unknown error occureed (-54)

= it stops while copying items photos and music. There is still GB space left on the mac......I tried all updates, restarted everything.

Can anyone help me how to solve the problem?

Thanks in advance

Rita