I tried closing the unfinalized disc with Imgburn,results are very similar to yours,but there is a problem,most probably the dvd-rw you finalized with Imgburn has the same problem...
Interestingly,when I open the "Dvd Decryptor" program the dvd writer stops triyng to read the unfinalized dvd,so I open that program to stop it.Then I opened Imgburn,chose build mode,and then Tools-Drive-Close_Disc.
Unfortunately,it gave the same error messeage which you captured and dvd drive stops reading and there is no busy hourglass icon.But,when I choose from the same menu "Close>Track" dvd writer did something.Then I chose "Close>Session" dvd writer again did something for a while,it's led keeped blinking,then when I open "Nero Discspeed" it show that the dvd was closed,but the problem is,unfortunately my computer couldn't read the dvd,couldn't play it,Windows Explorer couldn't show the dvd's contents.
Then I put the dvd into a standalone dvd recorder(that dvd was recorded with this standalone dvd recorder but was not finalized),and like you said,when I went to "finalization menu" ,"finalize" option was unclickable,so it was behaving like the dvd was finalized,though when I tried to play that dvd it had not a menu.
So I think although that dvd became closed,it isn't closed in a proper way and that's why it is not playable using my computer and most probably unplayable by other dvd players...
Can you try playing that dvd-rw which you closed by using Imgburn,with Windows Media Player?(if it's not an important dvd).
Also,for closing a disc with Nero,start "Nero Vision" then "Disc tools>Close disc(or Finalize disc)".