Problem adding images or links in IE only

We're having an issue when trying to browse the server to add a link or image. The problem is only in IE (which is what our client uses) - when you select the text and click the Link button, the window opens but seems to have more options than normal (see screengrab). If I type in a link then click OK, the window won't close. The close button top right won't close it either and the Cancel button also does nothing - in fact the only way to close the window is to hit the Back button, so of course the link doesn't get added. Same issue when trying to add an image - the image window won't close either.

I followed the instructions in another post to update CKEditor and add the meta info for IE9 but this hasn't helped. I have IE11 and I think my client is on 9 or 10 but seems to be the same problem on all versions.