No, the message is related to too many sockets (hence streaming clients), not thread.
However, this is not a Lightstreamer limit, but rather a system limit that you can enlarge.
Assuming you are on Linux, use the ulimit command and increase the maximum number of open files (provided that your login user is allowed to do that).

Note that it is possible that the message is issued in a strict loop until a socket is available again.
This is an undesired side effect that will be solved on the next Server release.