Drupal.settings.tabs is undefined

I'm not sure if this is related to Vertical tabs, but I haven't got his problem before enabling this module.

If I have a page which uses vertical tabs (e.g. edit profile, edit node), sometimes Drupal won't load ANY Javascript. IE7 says that there is an error and I get this when I open the console:
"Drupal.settings.tabs.speed error" is null or not an object

With Firefox the problem happens way less frequently, but using Firebug I get more info when it occurs.

Error: invalid object initializer
Source File: http://hlab.ee.tut.fi/piiri/user/register
Line: 63

Error: Drupal.settings.tabs is undefined
Source File: http://hlab.ee.tut.fi/piiri/sites/hlab.ee.tut.fi.piiri/modules/tabs/tabs.js?x
Line: 5

Looking into the first error I get this

jQuery.extend(Drupal.settings, { "basePath": "/piiri/", "tabs": { "slide": false, "fade": true, "speed": "fast", "auto_height": false ...

which continues for 20 lines or so. As you might notice, we have a multi-site installation of Drupal.

Looking briefly to vertical tabs code, I cannot see any value set for speed variable (which seems to be causing this problem), but I'm not a JS guru and don't know where the problem might hide.