Problems with upgrading

I had an older version of WP: 2.3.1, and I decided to upgrade to 2.6. But since the time I did it, there is an error message when I try to log in
Warning: Invalid argument supplied for foreach() in /var/www/kbx.cz/mentalove/wp-includes/capabilities.php on line 31

Warning: Cannot modify header information – headers already sent by (output started at /var/www/kbx.cz/mentalove/wp-includes/capabilities.php:31) in /var/www/kbx.cz/mentalove/wp-includes/pluggable.php on line 595

Warning: Cannot modify header information – headers already sent by (output started at /var/www/kbx.cz/mentalove/wp-includes/capabilities.php:31) in /var/www/kbx.cz/mentalove/wp-includes/pluggable.php on line 596

Warning: Cannot modify header information – headers already sent by (output started at /var/www/kbx.cz/mentalove/wp-includes/capabilities.php:31) in /var/www/kbx.cz/mentalove/wp-includes/pluggable.php on line 597

Warning: Cannot modify header information – headers already sent by (output started at /var/www/kbx.cz/mentalove/wp-includes/capabilities.php:31) in /var/www/kbx.cz/mentalove/wp-includes/pluggable.php on line 770

And another error is when I try to type wp-admin/upgrade.htm: Fatal error: Allowed memory size of 8388608 bytes exhausted (tried to allocate 11520 bytes) in /var/www/kbx.cz/mentalove/wp-admin/includes/media.php on line 213 . There’s also problem with czech diacritical marks. When I try to downgrade back to 2.3.1 the errors are still there. And there’s one new:Warning: Invalid argument supplied for foreach() in /var/www/kbx.cz/mentalove/wp-includes/capabilities.php on line 31 .
Now, I don’t know what should I do.
Thanks for your help.
Website: mentalove.kbx.cz