qm --display spice-app with ubuntu 26.04 and windows-11 stuck at "Connected to graphic server"

[bagong]

High, beginner here. I run quickmenu on a NixOS system and Hyprland (xwayland available). I have gotten both a Windows 11 and Ubuntu 26.04 running with --display sdl and --display spice. I can see a very noticable speed difference between the sdl and spice configurations. Also the virtio-vga-gl driver can match my native screen resolution, which is 2560x1600. I could use --display sdl, but I would love to have clipboard sharing and also love spices ability go scale to all window sizes. So I would love to get the --display spice-app option working. However what happens (both with Ubuntu and with Windows) is that a window comes up (remote-viewer ? (it looks different from the spice window)) displaying: Connected to graphics driver. But then nothing happens and after a couple of seconds the window times out, with a OS-Window coming up, reporting: "an application ubuntu-26.04 (1) - remote-viewer is not responding", and offers me to terminate or wait. After a couple of more seconds the window disappears. The "ubuntu-26.04" process and qemu are still running, I have to kill them to be able to restart the VM.

The log of such a failed attempt contains this:

❯ cat ubuntu-26.04.log
qemu-system-x86_64: info: Launching display with URI: spice+unix:///run/user/1000/qemu/ubuntu-26.04/spice.sock

(remote-viewer:173178): GSpice-WARNING **: 12:20:52.915: Warning no automount-inhibiting implementation available
qemu-system-x86_64: warning: console: no gl-unblock within one second
qemu-system-x86_64: warning: spice: no gl-draw-done within one second
intel: the execbuf ioctl keeps returning ENOMEM
qemu-system-x86_64: warning: Spice: Connection reset by peer
qemu-system-x86_64: warning: Spice: Connection reset by peer
qemu-system-x86_64: warning: Spice: Connection reset by peer
qemu-system-x86_64: warning: Spice: Connection reset by peer
qemu-system-x86_64: warning: Spice: Connection reset by peer
qemu-system-x86_64: warning: Spice: Connection reset by peer
qemu-system-x86_64: warning: Spice: Connection reset by peer
qemu-system-x86_64: warning: Spice: Connection reset by peer
qemu-system-x86_64: warning: Spice: ../server/reds.cpp:2278:reds_handle_link_error: Connection reset by peer
qemu-system-x86_64: warning: Spice: Connection reset by peer
qemu-system-x86_64: terminating on signal 15 from pid 183755 (htop)

I am quite a beginner in things virtual machines and qemu. I suspect however that things just fail at something rather trivial, a missing dependency, a wrong port or something alike. Do I need to install a driver inside Ubuntu. I did install the virtio-win package in Windows, and tried to install virt-vdagent and the webdav driver only to find that they were already there. But anything on the graphics side? Mesa? Something firewall, wayland or dbus related?

Thanks for any help!
Best
.r.