For my own record as well: C:\ProgramData\Microsoft\OfficeWebApps\Data\Logs\ULS

 

If you do not see C:\ProgramData folder, you need to change your explorer's viewing option to "Show hidden files, folders and drives".

Enjoy troubleshooting OWA!

Zewei Song, Ph.D.
Microsoft Certified Master: SharePoint 2010
Microsoft Consulting Services