fwiw I had (on sme9.2) upgraded Nextcloud to v21.01, yep living dangerously
It worked fine. It has migrated over to sme10 and is still working as expected but I mainly use it for managing a few file xfers between my laptop and mobile phone, calender and tasks.
I had the occasional issue on sme9.2 when an upgrade failed or timed out, the trick to resolve it was to run the update again from the command line. ie
OCC upgradeIf that reported that there was no update available and the web interface said there was an update then ignoring it for a day or so and trying again seemed to sort it out.
The latest update is 21.04 but I haven't gone there yet. Recovering to a previous version is a pain in the ass. Don't try it on a production server.
And yes it does throw warnings on the web update page about the database not being supported anymore.