We cannot render a new actionable message right now. Please close an existing one in Microsoft Outl

We have a user who gets this message:  "We cannot render a new actionable message right now. Please close an existing one in Microsoft Outlook and retry."

so, how is the user supposed to be able to find actionable messages.  The user does not even know what those are or how to find them.  I cannot find anything that explains how to find them - it looks like outlook has a limit of having 10 actionable messages, 
https://support.microsoft.com/en-au/topic/error-when-opening-actionable-messages-be7b8154-f293-4784-a3d9-08bef7f4c36e