If a balancer window is defined, and sh.startBalancer is run outside the window, then it will timeout after 30s (by default) with a cryptic error message. Better would be if sh.startBalancer noticed that a window is defined, and if the most recent entry in config.mongos indicates that the mongoses are outside the window, then it doesn't wait for the balancer distlock and instead prints a message. See also DOCS-6721.