My guess is that it's a server overload issue - DVC booking software is probably located on the same servers as the rest of Disney's reservation applications. So if too many people are trying to log on, some get denied.
DVC site & booking engine is working for me now (as of 8:20 am Eastern).
If they book a regular room they do. I actually thought about it, but I need all of my Disney funds for June and not really sure where I will be working come December, as to getting time off.