My session keeps timing out every 5minutes. I get this message:" Your session has expired. Log in again. (error15216460703184973

I get the following message:" Your session has expired. Please Log in again. (error15216460703184973).

This happens when I am in the middle of composing, or sending a message, it doesn't matter what I am doing, and this occurs. I have lost email replies etc. Very frustrating. When I look at the error message record, it says I have no error messages.

Please respond ASAP!! I have a business to run!