How are you accessing your email? With only the webmail interface and a browser? Or a desktop client like Thunderbird or Mac Mail? Something else? Exactly what happens when you try to Reply to an email? If using only the webmail-----------------
Try clearing out your browser's cache/browsing history. When you have done that, close out the browser and re-start it in a fresh session. If that doesn't do it, then try using a different browser.