Cant get to the theme page

I keep getting the following notices when I look at the theme administration pages (and some other admin pages too). This is only on one of my sites (I'm running multisite).

Notice: unserialize() [function.unserialize]: Error at offset 72 of 76 bytes in E:\www\vhosts\drupal\includes\bootstrap.inc on line 390

Notice: unserialize() [function.unserialize]: Error at offset 573 of 577 bytes in E:\www\vhosts\drupal\includes\bootstrap.inc on line 390

Notice: unserialize() [function.unserialize]: Error at offset 573 of 577 bytes in E:\www\vhosts\drupal\includes\bootstrap.inc on line 390

These errors have persisted, even though I upgraded the code to 5.6.

This same site has some other problems related to the zen theme, these all came up at the same time, so I assume that they are related:
Whenever I try to enter the zen subtheme configuration page:

Fatal error: Unsupported operand types in E:\www\vhosts\drupal\sites\all\themes\zen\greengrass_blueskies\theme-settings.php on line 45

Looking in the code, the offending line:

// Add the base theme's settings.
  include_once './'. drupal_get_path('theme', 'zen') .'/theme-settings.php';
  $form += zen_settings($saved_settings, $defaults);

What's more, when Opening the Zen configuration page:

    *  warning: array_merge() [function.array-merge]: Argument #2 is not an array in E:\www\vhosts\drupal\includes\theme.inc on line 278.
    * warning: array_merge() [function.array-merge]: Argument #2 is not an array in E:\www\vhosts\drupal\includes\theme.inc on line 278.
    * warning: array_merge() [function.array-merge]: Argument #2 is not an array in E:\www\vhosts\drupal\sites\all\themes\zen\theme-settings.php on line 38.

Does anyone know what's going on?
Thanks...