Hello Marcin,
There is an additional point about deleting an ECO. Could you please try to login client with Administrator role if the issue is in Productstream 2009?
Because there is an update about ECO deleting in PS 2009, System Administrator can delete any status ECO. An error dialog will pop up when Administrator role delete a Approve status ECO, please click ‘Detail >>’ button to expand that dialog to get 'Ignore restrictions (delete unconditionally)'checkbox, turn on the checkbox, then click Yes button, the ECO will be deleted.
If the ECO still can't be deleted by the Administrator, it may be a defect. Please follow Uffe Bak and Richard Rankin's suggestion.
Hope this is helpful.
Best Regards,
Carrie