ERR_QUIC_PROTOCOL_ERROR: The QUIC / HTTP3 connection failed

The browser tried HTTP/3 over QUIC and the connection failed. Often a network path that mishandles UDP, or a CDN advertising HTTP/3 support it cannot deliver on that route.

Short answer

The browser tried HTTP/3 over QUIC and the connection failed. Often a network path that mishandles UDP, or a CDN advertising HTTP/3 support it cannot deliver on that route.

What ERR_QUIC_PROTOCOL_ERROR means

QUIC runs over UDP rather than TCP. Networks that filter or rate-limit UDP break it, and because browsers advertise HTTP/3 via Alt-Svc, the failure can appear only after the first successful visit.

Causes and fixes, most likely first

1. A firewall or network filters UDP port 443. Allow UDP 443, or disable HTTP/3 advertising at the CDN so clients stay on TCP.
2. The CDN advertises HTTP/3 but a middlebox breaks the QUIC handshake. Turn off HTTP/3 at the CDN while investigating. The site will fall back to HTTP/2 with no visible difference to users.

How to stop it happening again

• Verify HTTP/3 works from multiple networks before leaving it enabled — its failures are network-specific and invisible from a single vantage point.

Check your own domain

An external probe confirms the site responds normally over TCP, isolating the fault to the QUIC path.

Frequently asked questions

Why does the error only appear on repeat visits?

Because the browser learns about HTTP/3 from the Alt-Svc header on the first response, then tries QUIC on the next visit.