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@diogobaeder so it does seem to be a GSettings schema issue due to the outdated packaged GTK3 lib. Apparently libqgtk3.so is only used for matching the Qt5 theme with the GTK3 one so maybe just setting QT_QPA_PLATFORMTHEME=kde or QT_QPA_PLATFORMTHEME=qt5ct can avoid loading this lib.

You can also try removing libqgtk3.so.

The other possible solutions I mention before are to remove the packaged GTK3 lib meaning libgtk-3.so* or to add those missing schema but the latter might break the XDG Portal and I'm not sure how this will affect the system.
I have the older schema installed and it works fine for me but my system is bare minimal and all the widget toolkit dependent apps I run through Flatpak.

Thanks a bunch, @tinywrkb ! So this is what I get: https://pastebin.com/XUgDw4yH - some missing antialiasing key, it seems... I'll try to tinker with that.

@diogobaeder, there's out.txt in .config/Insync and you can get a bit more verbose output by setting the environment variable QT_DEBUG_PLUGINS=1. Also, try starting with --no-daemon.
While the comment is now deleted, @rubusarbaro posted that the app is complaining about missing GSetting schema. Even though this is PySide2/Qt5 app, and not GTK3, I guess maybe it still makes some call to the GTK3 lib, so my suggestion about trying to add those schemas or removing the packaged GTK3 lib still applies, though I find it unusual that this would crash the app.

Not working on Gnome 40.0 for me either. If I run "insync start", I get nothing as output, and there's no verbose option. I tried using strace on it, but couldn't find a clue, it just seems to run /usr/lib/insync/insync but exit immediately afterwards. I don't know how to properly debug it.

Sounds like insync is aware of the issue and it will be fixed in the next release: https://forums.insynchq.com/t/insync-not-working-on-gnome-40/17149/3

I'm not opposed to removing more of the packaged libs since this is a huge package, but I'd rather defer to their fix.

@renatocrrs this shouldn't break the app, it should run even without that schema but with degraded font rendering.

This was introduced because those schemas were moved out from gnome-settings-daemon into gsettings-desktop-schemas.
If I understand correctly this change requires an updated GTK3 but Insync ships its own version.

I had a workaround for a relevant gsettings schema issue that I uploaded as an AUR package, gsettings-xsettings-schemas, but I'm not sure that it's safe to use after the Gnome 40 release so the AUR package is installing nothing now.
If you're brave enough, can work without a GUI if it breaks, I mean directly with the Linux console, then try building the package from here and report back. You can also just copy the schemas.
If it works, doesn't break XDG Portal, and you don't see errors about the schemas in the log then I can bring it back to the AUR for compatibility with older GTK3 releases.

@thenaterhood

You can probably remove the packaged GTK3 lib and let the app pick up from the system as the ABI is stable, but I'm not sure how safe it is.
In my Flatpak packaging for Insync I'm purging a bunch of libs that are already included in the runtime, see here.

@rubusarbaro can you be more specific about what you're seeing?

It's not working with Gnome 40