Can I resolve the 'OWA2 Server Core OWA User Not Found Exception' without modifying the OWA2 Server Core configuration?

Potentially, yes. If the issue is due to a temporary glitch or a problem with the Exchange Online service, simply trying a different OWA URL or attempting to log in again might resolve the problem. However, if the issue persists, it's likely related to the user account, Active Directory synchronization, or the OWA2 Server Core settings, requiring further investigation.