It seems I may have fixed this, although I hope the rest of you are using Ad Muncher too or it's a dead end
It probably has to do with an Ad Muncher advert list update or Microsoft changing the way their adverts work, i.e. killing the application if it detected an advert was blocked. Who knows.
Simply click msnmsgr in the Recent Program Names list and click on Don't Filter, it should then appear in the My Filter Targets with a "-" which means filtering is disabled on that process name. Hope this works for you!
edit: It still makes no sense why it has to crash with Plus installed, and fine without, until I added msnmsgr to the exclusion list in my ad blocking program.