Yesterday needed old picture from my nextcloud sailsync-backup and noticed that my phone also did stop syncing them already in April. (heh, long time not to check the backups 
)
First checked this @nephros way to fix that, but it seemed to work just like the original owncloudcmd version. So I did restore the original configs back to that.
Checked Nextcloud site about that client version from Which nextcloud desktop client version is working with which server version (server version newer than client version)?) - 💻 Desktop - Nextcloud community
As @nYK6zC said it is the Nextcloud 31 that defines that new minimum version and thus does not allow the connection. So if you have access to your Nextcloud servers configuration then the easiest way is to add the minimum allowed version definition to 2.5.2 there.
‘minimum.supported.desktop.version’ => ‘2.5.2’,
Sure this is not a correct solution either, but while waiting the Qt upgrade, there is no need to hack my otherwise working old sailsync app in the phone(s) at all. It works again and in the time of having a lunch I had again all my pictures up-to-date in the nextcloud server.