Bug#829180: updated to stretch, problem persists

[Daniel Pocock]

The systemd package from jessie-backports was installed, same problem

The whole system has been updated to stretch, the problem persists with
the systemd 230-7 package

As discussed in the upstream bug report, I've tested wrapping the mount
call with a script that uses strace, it shows that the mount call return
value is EBUSY

Can anybody suggest how to get more details next time the problem
happens?  It is reproducible on every 2nd or 3rd boot