Checking qa-page.php in v1.6.3 I see on line 734:
setcookie('qa_noticed', 1, time()+86400*3650, '/', QA_COOKIE_DOMAIN); // don't show first-time notice if a user has logged in
Line 224 also sets a cookie. Line 363 sets a header.
If this warning only appears in the admin/plugin section, you might want to modify file "q2apro-onsitenotifications-admin.php".
Because you have installed the plugin already, please try to remove the entire block of "function init_queries($tableslc) {...}" and see if the warning error still appears.
