---
title: "ERR_QUIC_PROTOCOL_ERROR: causes and how to fix it"
description: "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…"
canonical: "https://lemwatch.com/errors/err-quic-protocol-error"
source: "https://lemwatch.com"
---

# 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

- A firewall or network filters UDP port 443. Allow UDP 443, or disable HTTP/3 advertising at the CDN so clients stay on TCP.
- 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.


## Related

- [All website error codes](https://lemwatch.com/errors)
- [ERR_HTTP2_PROTOCOL_ERROR](https://lemwatch.com/errors/err-http2-protocol-error)
- [ERR_CONNECTION_TIMED_OUT](https://lemwatch.com/errors/err-connection-timed-out)
- [Free Uptime & response check](https://lemwatch.com/tools/website-uptime)
- [ERR_SSL_OBSOLETE_VERSION: The site uses an obsolete TLS version](https://lemwatch.com/errors/err-ssl-obsolete-version)
- [ERR_NETWORK_CHANGED: The network changed during the request](https://lemwatch.com/errors/err-network-changed)
- [Website monitoring overview](https://lemwatch.com/website-monitoring)