Error Notifications

I tried to install Contact Form 7 in WPMU installation but I got the following errors in one of the sites of WPMU network:

Warning: fopen(/home/*******/public_html/*******/wp-content/plugins/contact-form/contact-form.php) [function.fopen]: failed to open stream: No such file or directory in /home/******/public_html/*******/wp-includes/functions.php on line 3306

Warning: fread(): supplied argument is not a valid stream resource in /home/*****/public_html/*********/wp-includes/functions.php on line 3309

Warning: fclose(): supplied argument is not a valid stream resource in /home/******/public_html/********/wp-includes/functions.php on line 3312

Since I got these error notifications I deactivated Contact form 7 and deleted it from network but I am still getting these errors.

I am getting these errors in only one site and not the other sites in the same installation.

I am using Google Chrome.

And what is more weird is I am getting following two more errors when trying to login in firefox on top of three that I mentioned above

Warning: Cannot modify header information - headers already sent by (output started at /home/*********/public_html/*********/wp-includes/functions.php:3306) in /home/*********/public_html/*********/wp-includes/option.php on line 563

Warning: Cannot modify header information - headers already sent by (output started at /home/*********/public_html/*********/wp-includes/functions.php:3306) in /home/*********/public_html/*********/wp-includes/option.php on line 564

Sorry to bother you guys. I know it's not one of the WPMU dev plugins but I would appreciate if you can help me with this...