I have used mobidedrm to unencrypt many eBooks for many months now and have done so with no problems whatsoever. These two books I purchased from a bookseller I had never used before and this is the first time I have encountered this issue. My computer is running Mobipocket Reader, version 6.2 build 608.
The books were encrypted AND they had a password. The only way I could get mobidedrm to work was by sending the books to my Cybook and then by copying them back to my computer (as I described). Maybe this is something unique with
this particular vendor.