Warning message in Admin

I’ve updated a site that had not been updated for some time. Turned out to have multiple problems with themes and plugins that rendered blank admin screens.

Hove for now disabled all plugins, removed the old & incompatible theme and set the site to one of the standard themes. Everything appears to be working again except for one error message that I can’t trace. On all admin screens, I get the following error at the top of the screen (even above Screen Option tab:

Warning: call_user_func_array() [function.call-user-func-array]: First argument is expected to be a valid callback, ‘wp_admin_bar_sidebar_toggle’ was given in ***/wp-includes/plugin.php on line 507

(*** redacted)
Since no plugin is active any more, i’m wondering what is causing this.
Any suggestions how to solve this would be much appreciated.

Thanks in advance!