can't give you an exact answer, but I can give you a clue
Its all to do with user permissions - you changed the name of the PC, so the drive assumes its now attached to a different machine, so the backed up folders don't belong to what its attached to - so they aren't shown
You'll probably find that if you delve deep enough in one of the funny named folders the files will be there, though well hidden, Any new backups will be in a new heriarchy on the drive
When I've been faced with this in the past I've had to wipe the drive and starrt again, though there must be a better solution