Immediately after the usual message:
[ 120.171374] mt7601u 1-7:1.0: mt7601u_rxdc_cal timed out
and the system halted. It did it a couple of times, so I gave up and reinstalled the system. Was this caused by a corrupt system file or could it be related to recent updates to 19.10?
That’s a USB Wifi Driver issue. Doubtful though that would cause a system freeze though.
I presume this is 19.10? There was an update to firmware and kernel just after the release. You’ll need to keep an eye out for further freezes - if it occurs again try the earlier kernel in grub.
I might have caught the Wifi dongle with my foot  I have done this on occasion, but that can’t have caused this problem - which happened a couple of times.
I should have said that the version was 19.10. I don’t know whether there is a precise definition for ‘freeze’, but nothing appeared to happen from that point on, as nothing else appeared on the screen.
[update] No further problems since I re-installed the system and have regularly updated it.