Could not load folder contents Something went wrong. Please try again later.

Dear Sir/Madam,

We got "Could not load folder contents, Something went wrong. Please try again later." error when browsing to our Power BI Report Server. 

• We can "Manage folder", upload pbix files, create folder "blindly". 
• We can view the report with driect URL such as https://sitename/Reports/powerbi/PowerBI/dashboard.  We then can manage the report's properties such as updating Scheduled refresh. 
• But we can't see the folders.  We also can't update our reports.

Any idea what could be wrong here?

Our VM is Windows 2012 R2.  Power BI RS is January 2019.

Thanks,

Chiayu