customer session issue, 1.50 bug or i missed something ?

i installed 1.39 for a site, none issue of session
but for the sites installed zen cart 1.50, they get the same session issue

after the customer's session getting expired,
in 1.39, it is redirected to time_out
but on my sites of 1.50, if the customer clicks on any page of the site after session timeout, it's redirected to page_not_found

firstly i didn't think it's an important issue until the orders missed in admin even the customers have paid

what do fix the session issue?

anyone could tell me what MAY cause this error? all my sites 1.50 get this session issue
customer session expires, it should been redirected to time_out page, but it's redirected to page_not_found
even for the visitors who not login, if they left desk for a while, then they click on any page of the site, the page is also redirected to page_not_found

Those are unusual symptoms.

• What addons have you installed?
• In what ways is your site customized or different from a brand new uncustomized install?
• Post your site URL so we can see the symptoms directly

i install the same addons to the sites 1.39 (zencart default template), none problem. (including image handler for 1.39)
but all the sites 1.50 (zencart default template) get the same issue as i mentioned. (including image handler is for 1.50)

i'm going to try installing another 1.50 site without any addons, i'll tell if it would cause the problem