I get the following error messages when creating book pages. The pages are saved but not displayed in the book hierarchy. The messages:

warning: array_reverse(): The argument should be an array in /path/to/html/includes/menu.inc on line 275.

warning: Invalid argument supplied for foreach() in /path/to/html/includes/menu.inc on line 275.

warning: array_pop(): The argument should be an array in /path/to/html/includes/menu.inc on line 313.

warning: array_key_exists(): The second argument should be either an array or an object in /path/to/html/modules/node.module on line 628.

warning: array_key_exists(): The second argument should be either an array or an object in /path/to/html/modules/taxonomy.module on line 30.

Any hint is appreciated.