Error: Mailbox User does not exist.

When I try to modify any mailbox properties from HC panel, it throws an error that Mailbox User does not exist. I have checked from back-end and mailbox user does exist there but why is HC panel throwing this error? 

I am using Exchange Server 2016 with HC10 Panel.

This only happens if the Exchange Server Connectivity is cut off from the HC panel. Please re-enter the Exchange Server Credentials and check connectivity in HC panel server management interface. This issue can also arise if you have changed the Exchange Server password and you have not updated the new password in the HC Panel.
 
You can also check Exchange Power Shell connectivity with HC Panel by following this kb article that HC team gave me when I had the same issue a while ago: https://portal.hostingcontroller.com/kb/a223/how-to-enable-remote-power-shell-for-hc-in-exchange-2010-2013-2016-2019.aspx?KBSearchID=49850.