I also have the same problem. Not just with my regular site, to which I’ve done a ton of tweaking, but also on a fairly fresh install on a different domain. What happens is that I click on “send private message” and everything comes up fine, an email with name of the person I’m about to email, plus a text-entry box to the right of the name. If I try to type into that field, the first thing that happens is that my header image reverts to the original blue of the default buddypress install… and then if I click “tab”, the following html comes up in that field I just tabbed out of: “”. Odd.
This installation of WordPress only uses the BuddyPress plugin. No other. It’s a relatively clean install, again, with 1 plugin. My main site, however, also exhibits this problem.