If you run the OWA Monitoring Script from here, then you might see one of the following error messages below:
WARNING: [23:39:47.220] : The server returned the Outlook Web App error page.
WARNING: [23:41:56.638] : The server returned the Outlook Web App error page.
One possible solution might be to check if which Client Throttling policy was assigned to the taskuser mailbox inside the script mentioned above. Please keep noted here, that if you use a different Client Throttling policy for the taskuser and for your users this might mean that your users see issues but not the monitoring and via versa. So i would prefer to use the same Client Throttling policy for the monitoring which is used for the users.
The OWA functionality for the taskuser which is embeeded inside the script is not enabled.