Can't Log In – recently Installed PressBackup ..

Had no problems since creating site many months ago – recently installed pressbackup and carried out an immediate back up – site all ok and settings set to back up site to Amazon S3 weekly – just by chance went to work on site and cannot log in and site has code on it …

http://www.buckspropertymeet.com

error lines are
arning: unlink(/tmp/session_bca221a4cf052e3b555b876ada474b86) [function.unlink]: Operation not permitted in /home/coxjr/public_html/wp-content/plugins/pressbackup/.core/w2pf_session.php on line 47

and

Warning: Cannot modify header information – headers already sent by (output started at /home/coxjr/public_html/wp-content/plugins/pressbackup/.core/w2pf_session.php:47) in /home/coxjr/public_html/wp-includes/pluggable.php on line 897

any advise please….

thanks