You deleted the 900 seconds? That means you can't stay logged in. You would need to modify that via your database.
Do you allow users to login to your site via both
http://www.yoursite.com and just
http://yoursite.com? If so, then you MUST have your cookie domain (AdminCP > Settings > Options > Cookies and HTTP Header Options > Cookie Domain) set to ".yoursite.com" (no quotes, but note the period at the beginning) or your users will have problems. Also, make sure to tell your users to click the Remember Me button when they login. You will probably need to clear your browser cookies after making this change.