@Anonymous 

I went further with researching this issue and have found that if my NAS server is connected using AFP protocol:

in my case - in Finder, menu Go->Connect to server -> afp://PersonalCloud.local

AC 2018 cannot open Xref with XOPEN command ("Open Xref" from context menu).

But if I connect my NAS using SMB protocol:

in my case - in Finder, menu Go->Connect to server -> smb://PersonalCloud.local

XOPEN starts to work correctly in 2018 version.