Cause
The If-Match does not match the current etag of the object: it changed since the last read.
What to do
Read the object again, reconcile and repeat with the new etag.
application/problem+json
{
"type": "https://masnet.com/developers/errors/version_conflict",
"title": "Different version",
"status": 409,
"detail": "...",
"code": "version_conflict",
"correlation_id": "..."
}