Bittrex: Bitcoin wallet disabled until after BIP-91 is settled

Statement on the upcoming Bitcoin activation of Bitcoin Improvement Proposal 91 (BIP 91)

Bitcoin Improvement Proposal 91 (BIP 91, also known as Miner Activated Soft Fork) recently locked in over 90 percent of all mining hash power, signaling majority support for this proposal. BIP 91's lock in effectively makes BIP 148 (User Activated Soft Fork scheduled for August 1) obsolete and nullifies the chances of the Bitcoin network forking through UASF.

source: https://support.bittrex.com/hc/en-us/articles/115000761392