VBO and denied access for batch

I am using a modified version (have tried the unmodified version also) of the view "Content" (admin/content2) which shows the content of the website, but also a column for possible group belonging (I am using OG 7.x-1.5).

When I try to do an bulk update on a few nodes (have only tested with 1 node or 2 nodes) using the function "Modify Entity Values" the process stays for several minutes on the page "Performing Modify entity values on the selected items..." only saying something like "0 items of 2 processed". Then the page (/batch?op=start&id=188) shows an Drupal error message saying something like (I translate it):

An AJAX HTTP-error occured Result code for HTTP: 403 Error information follows. Path: /batch?id=188&op=do Status text: error Response text: Access denied  | [name of website] @import url("http://www.xyz.com/modules/system/system.base.css?mc4ri2"); @import url("http://www.xyz.com/modules/system/system.menus.css?mc4ri2"); ... [a lot more of the same]

What does this mean? Access denied? I am running as user 1.

In the error log in Drupal it says:

Type:	 access denied
Date: Friday, 19 October, 2012 - 11:16
User: admin
Place: http://www.xyz.com/batch?id=188&op=do
Referred from: http://www.xyz.com/batch?op=start&id=188
Message: batch
Grade: warning
Värdnamn: nn.nnn.nnn.nn
Functions:

I get no MySQL error concerning to low "max_allowed_packet" (the value is 8388608).

It works fine to do other batch jobs, like e.g. getting information about module updates.

I am using Views 7.x-3.5 and Chaos tool suite 7.x-1.2 (but for the moment only Drupal 7.12).