Really since Nov 28? Jeez, this is almost one week - which is incredibly long delay (and unfortunately it still persists). Maybe some troubles with XYM implementation (if that's the case) ? Isn't somebody from NEM/Symbol dev team in touch with Bittrex (more generally with all exchanges, these are trying to implement XYM support) ?

In any case I hope this maintenance will not last until launch of XYM will be ready (which will not be earlier than Jan 14) ...