Office Online Server – Known issue in ULS logging in Security Updates starting in May 2019

We have identified a new issue in Office Online server in which ULS logging is now set to verbose by default due to a product defect.

Though the logs are cleaned up automatically after a set amount of time, heavy usage could result in filling up the drives. If the drive that the logs are on is also the installation drive, this can cause an outage because the services will not be able to run.

This issue occurs in all updates starting with May 2019 and after. If you are experiencing a service outage, you can immediately resolve the issue by clearing the log files and restarting the service. You should set the log verbosity to medium to prevent the issue from occurring again. In order for this change to take affect you would also need to restart the Office Online Service.

Example (Powershell):

Set-OfficeWebAppsFarm -LogVerbosity Medium
Restart-Service WACSM

It is recommended that anyone who is updated to May 2019 or later set the LogVerbosity to Medium using the above commands.

Microsoft is aware of the problem and is working to determine whether a fix will be released.


  1. Stephan

    Was a fix released? I am planning to build an OOS farm with OOS Dec 2021 CU.

    v/r, Sue


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