I finally updated my Omnia 7!
- Tried about 4 times: failed
- Deleted all my music (around 8.5GB): failed
- Deleted all my apps except Engadget and Diodict: failed
Next steps should have been to delete games, then accounts, until I would be left with no choice but to do a factory reset.
Luckily I had TweetDeck tracking the error code in case something came up and surely, @dvroegop found a trick: He turned off Wifi then deleted all his pictures and managed to update straightaway.
At this point I had nothing to lose, really wanting to avoid a factory reset (didn't want to lose Engadget, Diodict apps and game saves) so I gave it a go (deleted videos too) and it worked first try
Could be pure luck but if you don't want to hard reset it's worth a shot