WHMCS Invalid backend configuration. Readable volumes not available.

I have an Ubuntu 16.04 server with ISPConfig 3.1 and a website where I installed WHMCS.

I'm trying to use elFinder file manager from the client area, but somehow it only works with some clients, because for most of the clients I get the following error message:

Code:

Invalid backend configuration.
Readable volumes not available.

Any ideas? How can I track and compare both cases?

 

I tried every possible thing, still same error.