Have you tried putting

Glib::Object->set_threadsafe (TRUE);
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at the top of your code?

If that dosn't do it, you are probably setting up your threads improperly, and you need to show a complete working snippet to demonstrate the problem. Gtk2 tries to be better at thread clean-up than Tk, BUT it is not perfect. If you try to dynamically create threads from anywhere in your program, you are quite likely to hit problems. Your thread code contains some complex ssh and sftp code, and they may not be threadsafe, or you need to do cleanup on the ssh/sftp connection before closing the thread. Try putting simpler code in the sftp thread sub, and see if it works. Like just connect, get a dirlist, and disconnect. If it works, then start adding complexity and testing as you go.