You were correct. Contribs updates were disabled (I have never touched these settings, so maybe a remnant of upgrade from some old SME version). I enabled it, and then issued
yum update --enablerepo=smecontribs
signal-event post-upgrade; signal-event reboot
(needed to fix shared group membership, because www is always removed after reconfig for some unknown reason, and reboot again)
It's a bit slow to start, but NextCloud now appears to be working. Eureka !
Once again, thanks again for your prompt help. I'm a bit of a dummy when it comes to administering some of this stuff, because during probably 15 years or more of using it, I rarely have to go beyond UI point and click - a real testament to the usability and stability of SME !