Outlook Graph Webhook Subscription Fails with "EXO Cafe Routing Error" for Specific Accounts

Question

We use the Microsoft Graph API (via Java SDK microsoft-graph 6.4.0) to create webhook subscriptions on behalf of users for mail and calendar events. Most accounts work fine, but for a few specific users, subscription creation fails with the following error:

com.microsoft.graph.models.odataerrors.ODataError: Operation: Create; Exception: [Status Code: InternalServerError; Reason: EXO Cafe routing error] at com.microsoft.graph.models.odataerrors.ODataError.createFromDiscriminatorValue(ODataError.java:36) ~[microsoft-graph-6.4.0.jar!/:na] at com.microsoft.kiota.serialization.JsonParseNode.getObjectValue(JsonParseNode.java:210) ~[microsoft-kiota-serialization-json-1.0.4.jar!/:na] at com.microsoft.kiota.http.OkHttpRequestAdapter.lambda$throwIfFailedResponse$0(OkHttpRequestAdapter.java:672) ~[microsoft-kiota-http-okHttp-1.0.4.jar!/:na] at com.microsoft.kiota.ApiExceptionBuilder.<init>(ApiExceptionBuilder.java:26) ~[microsoft-kiota-abstractions-1.0.4.jar!/:na] at com.microsoft.kiota.http.OkHttpRequestAdapter.throwIfFailedResponse(OkHttpRequestAdapter.java:671) ~[microsoft-kiota-http-okHttp-1.0.4.jar!/:na]

We receive this error during the POST /subscriptions call, even though:

• The same code works for other users.
• Permissions are identical (Mail.ReadWrite, Calendars.ReadWrite, etc.).
• Mailboxes are active and valid (users can send/receive).
• Token is a delegated access token.
• Subscription creation payload includes a valid notificationUrl, resource, and expirationDateTime