I decided to close all the terminals, including the one which launched roscore. I executed roswtf again, after roslaunch turtle_tf turtle_tf_demo.launch, and now I don't have the errors above anymore, but just a warning
What's happening? Why did roswtf think that certain nodes (e.g. /teleop) were supposed to be connected (to what? to the master?), even though some of them were not connected but some were?
Most likely the master that was still running maintained some stale information, which caused roswtf to conclude that certain nodes should be on-line while they weren't. The TCP ports it tries to connect to would then be closed, resulting in the errors you show.