Without knowing what the 2 clients are, hardware wise, or what processor the vmlinuz file was compiled for, I am guessing.
I have seen the 'Unable to load file' message when attempting to boot an i686 kernel on thin clients/old hardware that is not really i686 compatible, like many Via based thin clients.
If the working client is a Pentium or better PC, and the non-working client is some kind of thin client hardware, you may want to assure the vmlinuz file is compiled for a i386, i486, or i586, but not i686.
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