426 — Protocol upgrade required
Domain : HTTP
What does this error mean?
The server requires the client to use a different protocol or version.
Where can it appear?
This error may appear in application logs, client messages, monitoring systems or protocol traces. Keep the complete timestamp, request context and surrounding events before troubleshooting.
Likely causes
- Application state or request preconditions
- Client request headers or payload
- Server or WebDAV resource state
Checks and resolution
- Read the response body and relevant headers
- Reproduce the request with a minimal client
- Check application and reverse-proxy logs
Caution : Fix the underlying application or resource condition before relaxing safeguards.