Teams Bot Framework - error code - ConversationNotFound

Question

We are currently experiencing an issue with the Bot Framework when attempting to send messages to specific users. A couple of weeks ago, we began receiving the following error message for certain users:

Error: ConversationNotFound

Despite our efforts, we are still unable to send messages to these users, and the error persists. Given that the conversation ID is unique, we are unable to identify a valid reason for encountering this error and have been unable to find a resolution.

Request for Assistance:

Issue Description: The error occurs for specific users, preventing them from receiving messages.

Error Message: ConversationNotFound

Current Understanding: The conversation ID should be unique, which makes this error unexpected.

Resolution Sought: We are seeking guidance on the cause of this error and the necessary steps to resolve it so that affected users can receive messages again.

We use Rest Call to the conversation ID to post message.

v3/conversations/{conversationId}/activities

{"error":{"code":"ConversationNotFound","message":"Conversation not found."}}

final var resourceResponse = microsoftRestClient
        .post()
        .uri(uri, **ConversationID*)
        .contentType(APPLICATION_JSON)
        .header(WWW_AUTH_RESP, AUTHORIZATION_BEARER_TOKEN)
        .body(activity)
        .retrieve()
        .body(ResourceResponse.class);