X won't forward (ssh -X)

[Lars Jensen <jensen@scs.unr.edu>]

After switching to woody from potato, I'm having problems with setting
the display on a remote machine:

Both machine A and B below runs woody.

I ssh from machine A into machine B with ssh -X, and when I run any
X-application (like xclock) I get an instant error: 
"Error: can't open display"

I have tries ssh -X'ing into B from other machines also, and had no luck
with them either.

Strangely enough, when I let A and B switch roles, everything works
fine. I have set up both machines myself, and I intended the setup to be
the same on A and B, but something must be different in their setup?!
Could there be any settings on B that needs to be changed in order that
X be forwarded?

Any ideas what could  be the problem, or how to debug it? 

Thanks,
Lars.

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Lars Jensen, Truckee Meadows Community College, Reno NV 89512-3999. 
Tel: 775.673.7113 E-mail: jensen@scsr.nevada.edu