I'm having this same issue. It really only happens often with new users, because I create mobile accounts every time someone logs into a Mac, but it's really annoying and it's gotten me yelled at. It does however happen to people sometimes when their machine goes to sleep at lunch and then they try to log in to unlock their machine, will not unlock to save their lives. Reboot fixes the issue in both cases.
I'm running DC v5.5 (some machines might still be running v5.4, but this was supposedly fixed in 5.1).
Just last Friday I had a user that couldn't log in and unfortunately I was out of the office, so another employee who used to do IT was trying to help and it ended up wasting a lot of time (we get a lot of freelancers). I tell ya, having something as simple as LOGGING IN be so complicated and fragile (it is always breaking) is really frustrating. I've tried to collect logs for this before but was told "we aren't seeing any of the usual things we should see when a logon is happening" even with debug mode on. anyway, end rant.