I don't know. Bittrex didn't respond to our outreach.
I'm sure it had to do with the fork that happened after a miner suddenly pulled 1.2 exahash from Uno.
Now Bittrex joins a long list of former UNO exchanges:
Safecex.com, Shapeshift.io, Btcpool.exchange, Altquick.co, Bitebi9.com, Cryptsy.com, Coinbroker.io, Coins-e.com, Allcrypt.com, MintPal.com, Atomic-trade.com, Comkort.com, Coinedup.com, Coinex.pw, CryptoRush.com -- Bittrex
We had the same with XMG. Also many forks because of extreme sudden hashrates.
We talked with Bittrex and let the wallet in maintenance until all problems were solved.
This took about a month. Now XMG is out of maintenance on Bittrex for two weeks or so.