ReadyNAS NV+ V2 Device is Offline ... Solution
Even though I wasn't exactly sure where to post this information, I figured it was worthwhile posting my findings somewhere since I recently encountered the subject line problem and wasn't able to find any proposed solutions other than using an OS reinstall, Factory Default or opening a case with tech support. In my case, none of those worked.
I was setting up a NV+ V2 (firmware 5.3.12) for a relative and suddenly found myself not able to open the Admin Page from RAIDar 6.2. All I saw when trying to do so was the Dashboard with a popup notice "Device is Offline". After browsing the forum and reviewing all of the closed discussions, I attempted several Factory Default startups along with OS Reinstalls and was always faced with the same "Device is Offline" notice when trying to access the dashboard. Since I couldn't access the dashboard, downgrading the firmware wasn't an option.
For whatever reason, I finally tried to access the NAS from another computer and I was greeted with the login screen and was able to access the dashboard. However, after logging out and attempting to login, the Device is Offline message again showed up. Here's what I finally determined to be my problem.
In my case, this error relates to the browser cache or history. My primary browser is Firefox 53.0 and after clearing the browser history (checking all boxes and only clearing the last hour) I was again able to access the desktop. I DID NOT encounter this problem when usine IE to access the dashboard.
I imagine the different results when using different browsers is due to the fact that IE allows you to proceed to a site with an invalid security certificate (everyone recalls the notice you receive the first time you attempt to access your NAS, right?) whereas Firefox requires you to create an exception before allowing you to proceed to an insecure site with an invalid security certificate. The problem persists even if you choose to proceed in Firefox and do not check the box to Permenantly Store this Exception.
My suggested solution: Clear your browser history before attempting any other solutions if you encounter the Device is Offline error.
Note that I have only encountered this problem when attempting to access a device running OS 5.xx, not on my OS 4.xx or OS 6.xx machines when using the same browser/computer configureation.
Also note that since this seems to be OS specific, I would venture a guess that the solution will also apply to a Duo V2.