Hi Anouk,
Our helpdesk responded to you on the 12th of June.
Our dev team advised the error is coming from a local plugin: profiletheme, on line 30 of /local/profiletheme/settings.php: call to admin_category->add().
On the ticket we say:
It would be worth asking your technical team to take a look at the file as mentioned above. As far as I am aware, there is nothing installed in the 'local' folder by default and this is not mentioned in the theme documentation, so the assumption at the moment is there is some conflict with this plugin and the theme.
Can you speak to your technical team and keep us posted on how it goes?'
cheers,
George.