slack-ruby-bot: Bot does not start properly

Hi, sometimes when I start a bot via Bot.run log stops at line:

D, [2017-03-12T23:11:30.152904 #13321] DEBUG -- Slack::RealTime::Concurrency::Celluloid::Socket#connect!: WebSocket::Driver::Client

and nothing happens (bot is offline in a channel), if I restart (not always it succeeds after first restart) it goes ok and next line is:

D, [2017-03-12T23:12:00.512804 #13345] DEBUG -- Slack::RealTime::Concurrency::Celluloid::Socket#write: GET /websocket/JcusPEHqlaNbFYuplPuOAyhywrG2vsyCe4woI6OO-WHTcldHSrtPCrrPUhV4JOHbOUcDebnXB7iDhqYCZuuc0qiDrpK7OkGoUOsIqj6FelwJ8DipUMdkVkHMI-H1edIg7IVVnaOjKeMlLw0x7AuyOnqILO8NUMQm4WKoVd5uu-o= HTTP/1.1
Host: mpmulti-09sg.slack-msgs.com
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Key: 1BZifyP4N4SXHF2X4qPC0w==
Sec-WebSocket-Version: 13

Why it can happen?