User Can't Login

I have page with shortcode in it, title “Downloads.” When client goes to website and selects “Downloads” page, he is prompted to log in, as expected. Client logs in with proper username and password and presses enter. The “Downloads” page flashes but then redirects client to the login page again. Client cannot get to “Downloads” page.

https://ww.wp.xz.cn/plugins/sp-client-document-manager/