Varba, I'm in the midst of fighting the same issue for a client. I'm commenting so I can subscribe to followup.
We have in the dashboard:
(slightly edited to remove personally identifying path info)
Warning: array_keys() [function.array-keys]: The first argument should be an array in /wp-includes/theme.php on line 513 in the dashboard, which tells us we're running 3.0.3 and running upgrade.php tells us the database is current.
In themes we have:
Warning: array_keys() [function.array-keys]: The first argument should be an array in /wp-includes/theme.php on line 513
Warning: uksort() [function.uksort]: The argument should be an array in wp-admin/themes.php on line 73
Warning: array_slice() expects parameter 1 to be array, boolean given in /wp-admin/themes.php on line 94
Warning: array_keys() [function.array-keys]: The first argument should be an array in /wp-admin/themes.php on line 150
Warning: natcasesort() [function.natcasesort]: The argument should be an array in /wp-admin/themes.php on line 151
In Widgets We Have:
No Sidebars Defined
The theme you are currently using isn’t widget-aware, meaning that it has no sidebars that you are able to change. For information on making your theme widget-aware, please follow these instructions.
And in plugins it tells us we do not have any plugins installed (which is incorrect) even when they are installed (although at the moment I have them all removed).
Things I've tried with no success:
Manual Full Reinstall of all but wp-config (Although there is a wp-content/w3tc folder I can't get to delete)
Removing widget_text contents from the wp-options table (suggested by otto to affect a similar error that was at wp-includes/widgets.php)
Disabling magic quotes both in htaccess and php.ini
Removing all themes but TwentyTen from the host and verifying that the database understands that TwentyTen is the current template.
Client also had the host do a cpanel rollback for him which returned him to 3.0.1 but did not resolve the errors. I've since done the manual upgrade that has moved him to 3.0.3 again.
Commenting out the two lines for encoding from the wp-config file (which can be involved with a similar error but apparently not this one)
Support ticket with host filed to re-check all host file permissions and delete the wayward w3tc folder. Awaiting host and client followup.
Any help would be greatly appreciated