This error indicates that the server sent a WebSocket data frame with one of the 'reserved' bits (
RSV1) enabled. According to the specification (RFC 6455), reserved bits:
MUST be 0 unless an extension is negotiated that defines meanings
for non-zero values. If a nonzero value is received and none of
the negotiated extensions defines the meaning of such a nonzero
value, the receiving endpoint MUST _Fail the WebSocket
In other words, this looks like the server is using an extension that has not been negotiated with the client.
Or it might be a symptom of a different issue.
To make it possible for us to pinpoint the problem, would it be possible to create a communication log at LogLevel.Verbose (as described here) and mail the relevant part to us for analysis (after removing any potentially sensitive data)?
Alternatively, if there is any way we could reproduce this problem ourselves, please let us know. We would then look into the issue.