Delete permission discrapency in resource browser

Hi All,
My client has a custom editor acces policy. When they are logged in the want to remove an image.
when they doe this in the 'MODX::MODX Resource Browser' (the resource popup from the TinyMCE -> insert/edit image) the get a permission denied when trying to delete.
When they try to do this from the file tab they CAN delete the same files.
How is this possible? and how can i fix this?
tnx
Ralph