The CTO of Bitfinex and Tether, Paolo Ardoino, has just tweeted that the Bitfinex exchange has held up XLM withdrawals due to issues on the Stellar blockchain.
According to the announcement made by the Stellar team, there are some issues with the Stellar Horizon service. Some validators have dropped out of the network. The team is working to solve this problem.
The SDF horizon endpoint will be the first to restore.