The "no internet connection" message is strange, but it's possible that because of this, the download has been corrupted.
Try to remove the Addon Manager entirely and reinstall it, and be sure you don't see any connection errors.
If you see that message saying that bglmanx might not be safe, keep ignore it, always reply Yes to the question if you want to run it, and try several times, eventually do a Windows reboot and try again.
It's not a problem of the Addon Manager (of course, assuming the download wasn't corrupted), I get it sometimes for random modules (Level-D, Pmdg, etc), but a reboot usually fixes it.