So this one's been driving me insane for a while (especially since I figured it out a while ago and never documented the solution). On some systems I would only be able to get PsExec working half the time...the other half the time I'd receive this infuriating error:

C:\Users\CJ\Downloads>psexec \\chrlcltsbx803 -i -s cmd.exe


PsExec v2.11 - Execute processes remotely
Copyright (C) 2001-2014 Mark Russinovich
Sysinternals - www.sysinternals.com


Error deriving session key:
The system cannot find the file specified.

So after running ProcMon to determine what file "couldn't be found," I realized--I was running PsExec as my elevated administrative user without a profile. As in:

runas /noprofile /user:chrl\CJ cmd.exe

Unfortunately, this doesn't seem to work due to something with HKCU cryptographic keys.

Either way, the issue went away once running under my elevated account without the /noprofile switch. Hope this saves someone a headache.