The fact that the drive letter (and colon) appear twice is also quite odd. Is the trailing part of the path (public.gmane.org) really part of the path to the target file? It's also odd that the path would start in the temporary buffer directory -- if you use Insert -> Include File, even pointing to a file in another directory, I don't think LyX copies the file to the temp directory.
Can you post the full error message, plus the correct path to the target?
The path to the target is c:\lyxfiles\thesis\ch3.lyx
The full error message is
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Error: Cannot open file:
C:/lyx/tmp/lyx_tmpdir3780a04072/lyx_tmpbuf0/c:/lyxfiles/thesis/ch3.tex Invalid arguement
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Are you finding the target by browsing, or typing in the path directly?
The same happens whether I type the path, with / or \, or if I browse.
Does the DOS window "behind" LyX report anything useful?
No information in the DOS window.
