No mesh (802.11s) on ath10k-firmware-qca9888?

I am testing mesh on this device CF-E355AC which uses the ath10k-firmware-qca9888 driver.

I could not get it to work!

Latest OpenWRT 19 stable uses the CT firmware, which does not support mesh.
Latest OpenWRT 18 stable gives some error and the mesh on 5GHz doesn't work (but does on 2GHz)
I compiled a custom OpenWRT 19.07.5 with ath10k-firmware-qca9888 instead of the CT firmware and I get this in the log:

Sun Dec 20 03:29:21 2020 daemon.notice netifd: radio0 (3004): command failed: Not supported (-122)
Sun Dec 20 03:29:21 2020 user.notice mac80211: Failed command: iw phy phy0 set distance 0
Sun Dec 20 03:29:23 2020 kern.warn kernel: [  634.312271] ath10k_pci 0000:00:00.0: Unknown eventid: 36933
Sun Dec 20 03:29:23 2020 kern.info kernel: [  634.319772] IPv6: ADDRCONF(NETDEV_UP): mesh0: link is not ready
Sun Dec 20 03:29:24 2020 kern.info kernel: [  634.775199] IPv6: ADDRCONF(NETDEV_CHANGE): mesh0: link becomes ready
Sun Dec 20 03:29:24 2020 daemon.notice netifd: Network device 'mesh0' link is up
Sun Dec 20 03:29:24 2020 kern.info kernel: [  634.883003] br-lan: port 2(mesh0) entered blocking state
Sun Dec 20 03:29:24 2020 kern.info kernel: [  634.888632] br-lan: port 2(mesh0) entered disabled state
Sun Dec 20 03:29:24 2020 kern.info kernel: [  634.894528] device mesh0 entered promiscuous mode
Sun Dec 20 03:29:24 2020 kern.info kernel: [  634.899539] br-lan: port 2(mesh0) entered blocking state
Sun Dec 20 03:29:24 2020 kern.info kernel: [  634.905107] br-lan: port 2(mesh0) entered listening state
Sun Dec 20 03:29:24 2020 daemon.notice netifd: lan (1374): udhcpc: performing DHCP renew
Sun Dec 20 03:29:24 2020 daemon.notice netifd: lan (1374): udhcpc: sending renew to 192.168.1.1
Sun Dec 20 03:29:24 2020 daemon.notice netifd: lan (1374): udhcpc: lease of 192.168.1.44 obtained, lease time 86400
Sun Dec 20 03:29:26 2020 kern.info kernel: [  636.933511] br-lan: port 2(mesh0) entered learning state
Sun Dec 20 03:29:28 2020 kern.info kernel: [  639.013531] br-lan: port 2(mesh0) entered forwarding state
Sun Dec 20 03:29:28 2020 kern.info kernel: [  639.019235] br-lan: topology change detected, sending tcn bpdu

Mesh doesn't work, the interface is brought up but does nothing.
I tried it with other devices using mt76 and ath10-qca8988x and I can get it to work.

The problem must be here:

Sun Dec 20 03:29:21 2020 daemon.notice netifd: radio0 (3004): command failed: Not supported (-122)
Sun Dec 20 03:29:21 2020 user.notice mac80211: Failed command: iw phy phy0 set distance 0
Sun Dec 20 03:29:23 2020 kern.warn kernel: [  634.312271] ath10k_pci 0000:00:00.0: Unknown eventid: 36933

What could I have done wrong? Does this ath10k firmware support 802.11s?

Mesh is supported and I think 802.11s also. You won't probably have the option to select in LuCI if not.

But it has issues it seems and I wonder why ct-version should not support mesh:

I have no clue about mesh. The errors you marked as the problem are IMO not the key issue here. I would ignore them for the moment (I doubt that they are relevant) and search the error in the configuration/setup.

Sun Dec 20 03:29:26 2020 kern.info kernel: [  636.933511] br-lan: port 2(mesh0) entered learning state
Sun Dec 20 03:29:28 2020 kern.info kernel: [  639.013531] br-lan: port 2(mesh0) entered forwarding state
Sun Dec 20 03:29:28 2020 kern.info kernel: [  639.019235] br-lan: topology change detected, sending tcn bpdu

Looks good to me.
Something other must be blocking communication. I cannot help further. Maybe another one knows more. A starting point for others would be the output of:

cat /etc/config/wireless