MacOS Sierra and ReadyNAS NV+ cannot find each other

Since upgrading my late 2009 iMac to Sierra, the connection to my beloved ReadyNAS keeping dropping off or will not connect at all. I can find the NAS via RAIDar and can also access media files via Apple TV ver 1 device (XBMC media player). I can 'see' the NAS via the browser URL. The NAS also appears in Finder in the 'Shared' section (NAS, AFP, CIFS options) however none of them gives me access to the files. Same issue when attempting to access NAS with MacBook Air (also upgraded to Sierra). I had none of these issues with the previus MacOS version. 

This is really really frustrating. Any advise would be most appreciated. I would hate to dump this great NAS because of what might be a trivial issue. HELP!! Many thanks.