Hi,
a quick fix for that, could be:
in 'application/views/mail/add_account.php' replace line 199 with this one:
<?php echo yes_no_widget('mailAccount[del_mails_from_server]', 'mailAccountDelMailFromServer', $del_from_server > 0, lang('yes'), lang('no'), 60) ?>
and in 'application/controllers/MailController.class.php' replace lines 800 and 962 with this one
if (!array_var($mailAccount_data, 'del_mails_from_server', false)) $mailAccount_data['del_from_server'] = 0;
this will be fixed for next release.
regards.