If using Quick Reply it will time out after a while ie if the thread has been open for a fari while, the quick reply will not work.

If you have typed in a reply and it indicates it has timed out.
1. Hit back until you see your reply again
2. I usually select the text, and copy it - on firefox it is right click and copy. I don't paste it anywhere. It is only a precaution and usually not necessary.
3. Hit refresh
4. The thread should reappear and and the text SHOULD still be there.
5a. If so, you can now hit ENTER and no problems.
5b. If the text has disappeared, right click and PASTE in the quick reply box and hit ENTER.

If using the proper reply box, no idea why it is timing out. Never happened for me.

Your problem sounds like the quick reply box at the bottom of the threads.