It started today? This has been a long-term ongoing issue with the way quick replies are done in threads.
It has to do with a php.ini setting, not much can be done about it. I believe vb gets around this by not making the quickreply box available unless you click on another button or link, it would be similar to refreshing the page when you are ready to type if you've been reading a long time. Recent threads versions do help in that if you back up to the previous page, your text is still there allowing you to copy, refresh the page, paste and submit.
The other issue the gives that error is that the quick reply links within a topic only work with the last post in a topic. If a different link is used you'll get the 'form is no longer valid' error. It hasn't been fixed yet. My fix was to remove the link from showflat. Those viewing in flat mode just use the box at the bottom and not choose a particular post to quick reply to.
