>The culprit _may_ be my window manager (fvwm95) which was
>built back in 2002 (1.6 ?)

I don't think so, I just tried again using twm that came with 3.1
and emacs died with a memfault when doing the actual paste,
but it did get past the mouse positioning the cursor - which reliably
killed it with

X protocol error: BadWindow (invalid Window parameter) on protocol request 38

when running fvwm95.

--sjg