It sounds like an inactivity timeout which may occur if you take your time writing an email. If it was ok a few weeks ago and it has just happening it the issue could be from AOL servers or your web Browser.
I'm assuming you are using the Windows 10 Edge browser, so to test if it is you browser or AOL try downloading and installing a different web browser such as www.opera.com or www.firefox.com. If it works ok in another browser then it is a good guess that its your edge browser or the add-on modules that it is using.
Alternatively, if it is Edge that you are having the trouble with, try clearing out the Browser Cache and Cookies, see:
https://www.wiknix.com/how-to-clear-cache-and-cookies-in-microsoft-edge-browser/
A work-around is to compose the message in Word Pad or Notepad then copy and paste the text into a new mail message, so you don't get cut off half through typing!