Welcome to MP.
Her is what a member posted about the problem.. Per Hackerman:
There are a number of reasons why Tor will log you out. Primarily it boils down to the IP changing and cookie handling.
A couple things you can try.....
In the TOR browser go to NO SCRIPT OPTIONS>ADVANCED>HTTPS>COOKIES and check the box that says, "Enable Automatic Secure Cookies Management".
Another thing you can try is....
TOOLS>OPTIONS>PRIVACY .. Under the HISTORY box use the drop down to select USE CUSTOM SETTINGS FOR HISTORY.
When the new options pop up check the box that says, "Accept cookies from sites"