If anyone has been having trouble with their spacebar not working in PM's, or even in the quick reply box, it seems as though it is a known Chrome specific bug with vB4 that the vB devs will not fix. You can get around this a few ways, most notably by writing your message in notepad and c/p'ing it over. Sometimes refreshing the page will work, or hitting enter before you start writing. Apparently this is most problematic when replying with a quote.