Cannot see admin pages after upgrade.. need to change theme, i suppose..

hello all,
followed all the instructions to upgrade from 6 to 7, but i suppose i didnt changed the admin theme because:
i can see my site, with garland theme, but when i try to switch to normal operation (instead of maintenance), i can see nothing...
for example, in http://my site/admin/appearance i see:
Catchable fatal error: Argument 1 passed to drupal_attributes() must be an array, string given, called in /var/www/vhosts/xxx.xxx/httpdocs/includes/theme.inc on line 1672 and defined in /var/www/vhosts/xxx.xxx/httpdocs/includes/common.inc on line 2295
and of course i see no content...
i suppose i have to chenge the admin theme, or something...