True, but stranger things have happened when a program runs out of RAM. In AE, I've had expressions spontaneously stop working, 30 lines that suddenly go missing from the end of an expression, odd errors out of the blue, complete wipe outs....
It's possible that the AE gets its RAM in a tangle and the memory address of the key frames in question somehow becomes corrupted.
I can't think of any plug-in that specifically requires that there be keyframes. Properties all have an implicit value whether keyframed or not. Likewise, an error in an expression should simply fail to compile and give an expression error. It also shouldn't be intermittent. It either compiles or it doesn't.
That's why it's such an odd error - and given the minimal information about it on-line, a rare one too.
As Mark pointed out, the fact that it only happens on some machines would suggest that it's something specific to the set up of that machine. When it's something to do with hardware, RAM's gotta be the prime suspect.
I don't say that's definitely what it is, but it's worth checking out.