The SRU for Xenial (bug 1598300) is a one-line patch removal on the CUPS daemon, the problem observed here and in many other reports is on the shutdown of the old CUPS daemon. So it seems that some independent change, in a not yet known package, causes problems shutting down CUPS, but the problem is revealed by the fact that an new CUPS version (2.1.3-4ubuntu0.1 from the SRU bug 1598300) got uploaded and so all users who have activated xenial-proposed get a CUPS update triggering this problem.
The SRU for Xenial (bug 1598300) is a one-line patch removal on the CUPS daemon, the problem observed here and in many other reports is on the shutdown of the old CUPS daemon. So it seems that some independent change, in a not yet known package, causes problems shutting down CUPS, but the problem is revealed by the fact that an new CUPS version (2.1.3-4ubuntu0.1 from the SRU bug 1598300) got uploaded and so all users who have activated xenial-proposed get a CUPS update triggering this problem.