After adding several Domain Name Servers on machine #1, I did a backup. I then did a fresh install of Core 141 on machine #2 and restored with the backup from machine #1. Everything went as expected but the Domain Name Servers did not get restored onto machine #2.
This is a known bug. The server list is not in the backup with core141.
Should be fixed in core142 (currently only in unstable tree) the version in testing is still affected.