Wow, thanks for that!!
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Can this issue be reproduced also on newer firmwares that is available with ADH v0.107.52?
https://dl.gl-inet.com/router/mt6000/open
It looks like the issue is related to the packet size of HTTP3 and QUIC of the new version ADG.
And the default MTU for WG client interface should be 1420.
As long as the MTU value is not manually changed (1420 > 1310), the HTTP3 and QUIC traffic will be normal.
Well, it's not that simple because I am using a cellular connection as a failover and the standard 1420 MTU for Wireguard simply doesn't work. There are at least 3 operators here in Italy (Wind, Postemobile, Very mobile) that gives problems when using 1420 MTU for Wireguard, I extensively discussed this topic when I was doing the beta testing with Spitz AX.
thanks a lot! I have just same issues with Quic via Wireguard, and its eat my brain to find solution.
I come back to this thread to try find solution again.
On my old Flint with 4.65 FW its work just fine even on old Adguard(forgoted version).
But now i update to Flint 2, add my Warp config and have this problem, update Adguard (from 52) to last version by script from @admon 
to v0.107.55 but its not help.
@bruce
Change MTU to 1420 is help thank you! 
In my Warp config default is 1280, so its not trivial things 
I know that use Quic is not big deal, DOH and DOT work just fine. But this error is annoying me and as i see Quic DNS is really work and respond match quicker lol
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