I got that error about an hour ago. I completely uninstalled the old beta client days ago and I'm running the launcher in admin mode. I've restarted the download but now it's downloading at a snail's pace. Terribly slow.
I posted this problem on the beta forums several times and several more people had the problem too, but it never recieved attention. The odd thing is that it would sometimes work and other timed throw the error and make you re-download everything. Trying over and over again seemed to be the only effective way to get around the problem.
I really feel this problem will make it to release with no official word from SE as to what is happening.