Undefined offset in capabilities.php when trying to edit a post

Hi,

I have just moved my multisite from one server to another, changing the site domain along the way. I followed the steps for moving a multisite described in the WP Codex and after some fiddling about everything seems to work as expected now… apart from one thing:

Whenever I try to create a new page/post or edit or update an existing page/post I get the following debug message and I am not redirected to the page/post edit page:

Notice: Undefined offset: 0 in /x/y/z/wp-includes/capabilities.php on line 1067

Warning: Cannot modify header information - headers already sent by (output started at /x/y/z/wp-includes/capabilities.php:1067) in /x/y/z/wp-includes/pluggable.php on line 876

I have already searched the forums and Google but none of the offered solutions seemed to work.

Interestingly, the URL I am redirected to when hitting the Publish/Update button seems to lack all arguments:

http://mysite.com/wp-admin/post.php

Another interesting thing is that the updates I do are saved and stored in the database. So when I visit the actual site any updates are reflected, but somehow the update process on the admin site is broken. I’m kinda stuck here, so any help would be greatly appreciated.