I'm getting this issue as well. Oddly enough, it's only when editing pages which include a template. The Visual Editor will work fine on any page without a template, or a page I'm currently adding a template to and haven't saved yet. Using the source editor to remove a template from a page will allow me to successfully jump into the Visual Editor and continue editing. Saving the page without a template will allow me to use the Visual Editor on subsequent edits. Swapping back and forth between the Visual and Source editors when removing an offending template, without saving the page, will eventually start to spit out HTTP 500 errors again.