I just upgraded my forum from 3.0 to 3.0.1 and am having some strange errors. If you clear the cache in the admin panel, when you go to the board index you get the following errors only if you are logged in. Once the page is refreshed, the debug notice goes away and will not come back until the cache is cleared again. This happens with any type of user (admin, normal).

Errors:

[phpBB Debug] PHP Notice: in file /includes/auth.php on line 805: Invalid argument supplied for foreach()
[phpBB Debug] PHP Notice: in file /includes/functions.php on line 3533: Cannot modify header information - headers already sent by (output started at /includes/functions.php:2956)
[phpBB Debug] PHP Notice: in file /includes/functions.php on line 3535: Cannot modify header information - headers already sent by (output started at /includes/functions.php:2956)
[phpBB Debug] PHP Notice: in file /includes/functions.php on line 3536: Cannot modify header information - headers already sent by (output started at /includes/functions.php:2956)
[phpBB Debug] PHP Notice: in file /includes/functions.php on line 3537: Cannot modify header information - headers already sent by (output started at /includes/functions.php:2956)

Also, if I edit a forum in the backend, click submit and then click "go to previous page" the following errors come up and the previous page is not displayed.

[phpBB Debug] PHP Notice: in file /includes/auth.php on line 805: Invalid argument supplied for foreach()
[phpBB Debug] PHP Notice: in file /adm/index.php on line 149: Cannot modify header information - headers already sent by (output started at /includes/functions.php:2956)
[phpBB Debug] PHP Notice: in file /adm/index.php on line 151: Cannot modify header information - headers already sent by (output started at /includes/functions.php:2956)
[phpBB Debug] PHP Notice: in file /adm/index.php on line 152: Cannot modify header information - headers already sent by (output started at /includes/functions.php:2956)
[phpBB Debug] PHP Notice: in file /adm/index.php on line 153: Cannot modify header information - headers already sent by (output started at /includes/functions.php:2956)

See this thread for discussion: http://www.phpbb.com/community/viewtopic.php?f=46&t=885115

Note: This was one of 10 forums I updated and this is the only one with issues. Everything is the same in all forums besides template colors.