Unknown error in Nextcloud client

We’ve got Nextcloud 11 running. Our users have been accessing it through the Apache reverse proxy. However, they all wanted to have the desktop client so I started looking into it.
As our reverse proxy requires client-side SSL certificate, accessing the reverse proxy URL within the client won’t work. So I’ve got it working so the client can access the direct URL. Though I’ve a problem. Once I finish the client setup I get error “An error occured while opening a folder Unknown error”. I’m getting the same on both Windows and Mac clients.
I’ve searched Google but didn’t get anything relevant to my issue.
I’ve setup a test environment with the latest version of Nextcloud and I’m having the same issue.
I’m clueless now.

Anyone has got any idea?