app api call slowdown, QUIC is suspected

It was reported that one of our Android apps was experiencing delays and timeouts in api calls, which hit a dot net backend running on Linux. Apparently the errors they saw were
 net::ERR_QUIC_PROTOCOL_ERROR

I suggested that they disable QUIC in their browser, https://thegeekpage.com/err_quic_protocol_error/

But apparently the app was having the errors and not in a browser. Since I'm not a dot net or Android developer, I'm not sure how one can disable QUIC in a generalized manner. But I could configure the server's firewall to explicitly Deny QUIC requests (UDP requests on port 443) which would then immediately make the client fall back to HTTP/2 (TCP packets).

There was an update pending on the server. Completing that and rebooting, still the issue continued. "For the past week, we've been experiencing buffering and delays in the app, especially on 3G networks". Maybe there was some change in either the network or Android or something like that? Cause could be any of those. Anyway: set up a firewall rule to Deny UDP to ports 80 and 443, and also disabled QUIC for the entire domain in cloudflare, https://developers.cloudflare.com/speed/optimization/protocol/http3/

That seems to have solved the issue.