A request was made to re-open a Session that would result in one or both of the Participants being active in multiple Sessions. This could result in calls or messages from the affected Participant being routed to an unintended recipient.
Multiple Sessions in your service have a Participant with the same identifier/proxy-identifier pair, and one of those was active at the time a request was made to re-open one of the closed sessions.
Avoid re-opening sessions unless you can be certain that the Participants have not been assigned the same ProxyIdentifier in other sessions.
Ask that the ProxyAllowParticipantConflict flag be removed from your account. By default, Proxy now rejects requests such as this one that would result in having two participants in active sessions with the same proxy identifier. However, this new behavior requires removing the ProxyAllowParticipantConflict flag from your account. When you are prepared to handle 400 on Session update requests (as well as 409 from Session create and Participant create endpoints), you can request to have this flag removed.
Include the experimental form parameter FailOnParticipantConflict=true in your Session create/update and Participant create requests to allow Proxy to reject the individual request if it would result in this condition. This will allow you to test your code before choosing to remove the ProxyAllowParticipantConflict flag described above.