My DS415+ Synology NAS has just been updated to DSM 6.0
In my Mac Book Pro running OS X El Capitan 10.11.4 the NAS was appearing in Finder but when I clicked on it the finder pane showed “Not Connected” or “Connection Failed” and clicking the connect as… button didn’t pop up window.
My fix was to do the following and reboot:
# from a terminal mv ~/Library/Preferences/com.apple.loginitems.plist ~/
After the reboot I could still see my NAS in Finder but when I clicked on it. The shares appeared in the right hand pane and I could now open them.
This may have (not really sure) fixed an issue I had where Time Machine was complaining about not being able to connect to the Time Machine volume on the NAS.