In order to enable verbose log on Office 365 Endpoint, proceed with the next steps:
1. Login to Office 365 Endpoint
2. Go to folder C:\inetpub\wwwroot\Office365Application\
3. Edit file web.config by changing "RollingLogFileAppender" value from INFO to DEBUG
Verbosity can be changed on the fly; no web site restart or pool recycle is required.
Do not forget to move the changes back when the investigation is completed.