How are you accessing your email account, is it on a browser and if so which one?
If on a browser how are you reading the email. Is it by double clicking on it and it opening in a new window or is it by single clicking and the email opens or is it by using a "preview" view and the email is read when you click on the email in the list and it shows either to the right or below the list?
I have check by double clicking and opening the email in a new window and I can reply without problem and close the window without problem so it would not appear to be a general problem.
As has been mentioned, try a single click and use the reply to arrow at the right hand side to see what happens.