I have a
build of the latest(?) a newer version of the official nextcloud client built against Qt 5.15 here:
https://build.sailfishos.org/package/show/home:nephros:devel:qt515/nextcloud-client-qt515
If you can, please test this against your nextcloud instance.
There should be a way to make sailsync use that binary through some clever symlinking or a wrapper script.
Unfortunately the nextcloud upstream source tree is broken in a peculiar way that makes it hard to clone older revisions of it, and this breaks building on chum.
Also, upstream dropped qt5.15 a few versions ago, so this one is the only one possible on SFOS right now.