Mhhh, we have to find out what is throwing the error message. To explain: Above the messages say, it is tried to set a cookie and output a header, problem is that the plugin sent already headers (an output). Question is, what (error?) has been sent.
Ah, I just see now that the file responsible seems to be q2apro-onsitenotifications-page.php regarding your warning message. But I cannot see a problem there since the file has a default structure. I try to check on this tonight again.