Symptoms

When the chat disappeared at end user side, you may observe below error on browser console,

Conversation <sys_id> does not exist.

While opening the disappeared conversation in connect mini window, you may observe below message,

security prevents viewing this conversation.

Release

Jakarta 

Cause

Custom business rule was updating the opened_by field of chat_queue_entry record with agent sys_id and hence Chat logic fails. 

Resolution

Review the custom business rule and set the condition of business rule as "sys_class_name != chat_queue_entry" so that business rule will not be executed for chat_queue_entry and hence chat logic would work as expected.