A user who is a member of the assignment group for a Connect Support Queue will not see that queue in the sidebar and have no way of accepting new incoming chats. This issue will most likely be intermittent and may last a few minutes and through multiple screen refreshes.
This issue may also impact all Chat agents at the same time for a particular queue or all queues.
The most likely cause is that the group record (sys_user_group) that is set as the assignment group for the queue (chat_queue) is inactive. If that is the case, you can set the group to active and it should fix the issue.
Here is a more detailed explanation.
A before query Business Rule on the sys_user_group table is preventing those users from querying the assignment group of the chat_queue record for that queue directly. A business rule exists on that table OOB and the only thing it adds is active = true. Therefore if the group is inactive you can face this issue. However, if that Business Rule is edited or if another business rule exists then it would need to be edited to make sure the user can see that group record.
The reason that the issue is intermittent is that we cache group membership and check that cache for membership of queues in Connect. So when a user, who can query the group, views the group, the agent's membership in that group also gets cached. If the cache is cleared, then the agents face the issue and won't see the queue in the chat panel, which is why this was so intermittent.
Make sure that the user is a member of the assignment group for the chat queue, and that the user can query that group correctly. This can be done by going to sys_user_group.list and searching for the group as that user.