IDL and "X connection to *:11.0 broken (explicit kill or server shutdown)"

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I get the error message

X connection to [server name]:11.0 broken (explicit kill or server

but ONLY when I try to run any IDL (Interactive Data Language) command
that displays graphics.

I do not obtain any error message when I run xclock or the gimp; these
display graphics just fine.  So there seems to be something wrong with
IDL in particular.  Any thoughts?  What can cause this error message,
without causing X11 forwarding to fail altogether?

I am SSH'ing to a remote machine using Hummingbird's Host Explorer,
and running Exceed.  I have enabled X11 port forwarding (under
Connection > Secure Shell in Host Explorer 13.)

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