Not having UE I assumed you meant to go in through the form builder interface. Did so. checked all parameters and "Finished". Uploaded all changed files.
Sent a test out and no one saw it except me.
This is an ongoing problem that occurred around the time they changed over to gmail pro. Also around that time the forms I was receiving (I receive them all as BCC) contained the following or something very similar (in private).
Did some checking and the general consensus was that it had something to do with php5 and the workaround was to include something like
<?php date_default_timezone_set("America/New_York") ?>
I included it at the top of that page and the error no longer came through.
Around 2:30 Eastern I sent out a test form after making the changes you suggested. Came to me only with the error (private). Inserted <?php date_default_timezone_set("America/New_York") ?> at the top of mail_php.php. Came back to me only without the error.