Files tab empty!?

Hello,
Setup MODX Revolution 2.2.0-pl2 for a few sites now and 2 of them are getting this issue.

When I look at the "Files" tab in Modx it is empty, nothing displaying.

See attached jpg.

Strange how this is happening on 2 sites and fine on the others. They are all hosted on the same server.

Any helps would be much appreciated!
Thanks