Problem after user joins

I ran a test and joined my site after I joined I got the following error, whats wrong and How do I fix it

Warning: fopen(/home/username/public_html/cache/user28.php) [function.fopen]: failed to open stream: Permission denied in /home/username/public_html/inc/profiles.inc.php on line 659

Warning: fopen(/home/username/public_html/cache/user28.php) [function.fopen]: failed to open stream: Permission denied in /home/username/public_html/inc/profiles.inc.php on line 659

Warning: fopen(/home/username/public_html/cache/user28.php) [function.fopen]: failed to open stream: Permission denied in /home/username/public_html/inc/profiles.inc.php on line 659
Join complete
Congratulations!

Your account has been successfully confirmed and activated.
You can log into your account now.

Also the errors appear on the home page as well with the addition of this one, the site has been working fine until today

Warning: Cannot modify header information - headers already sent by (output started at /home/username/public_html/inc/profiles.inc.php:659) in /home/username/public_html/inc/design.inc.php on line 495

OK I figured this out on my own I read the error code a few times and "Permission denied" stuck in my head, I decided to check out the cache folder for user28.php and to to my surprise it was not there. I also had two other users not there user29.php and user30.php, I thought to myself they are not being created during the signup process, I looked at the permissions of the cache folder and set them to 777 loaded my website and it loaded error free and the 3 user.php files were created

NOW I thought during the install process I had to change the permissions of the cache folder to 777, how would the permissions change by themselves 

Please advise thanks

Set the permission of cache folder to 777.

Those files will be deleted and created by dolphin. Don't worry about it.