I am experiecing the same behavior, I have a pair of msm765 controllers. I am currently at 6.4.0.0.
Were you able to resolve this issue? Its still down for me and I have 13 access points down with it. I created a case but their suggestions was to download and install 6.4.1.0.
I read the release notes and I just dont see how this update will fix it? I plan on updating the controllers tonight but if there is anything at all that you could share, I would really appreciate it.