I'm not sure this is a related issue, but do you have more than one Mobi device registered for your ebooks? For example, do you have your desk top computer, a PDA, and a Cybook installed (just as an example - don't have to be exact combination!) You might try one of the other device's PIDs in that case.
The other thing I regularly "hiccup" on is that you either must have a filename (and new filename) with no spaces in it, or you have to put the filename in quotes.
So using
Code:
mobidederm05 Old Filename New Filename PIDnumber
Might give a bad length error message because it thinks "New" is the PID.
Instead, it should be
Code:
mobidederm05 OldFilename NewFilename PIDnumber
or
mobidedrm05 "Old Filename" "New Filename" PIDnumber
I'll admit, I've never tried using the quotes. I just take any spaces out of my filenames. I can always rename them later.