Blank pop-up box when selecting Visual Editor icons

When I click any icon on the Visual menu bar which normally would cause a pop-up window to show (eg Paste as Palin Text, Paste as Word, Insert Custom Character etc) a plain white box appears with nothing in it.

See screenshot at https://i53.tinypic.com/344sjdw.jpg

I’ve tried adding TimeMCE Plug but still doesn’t work. Same for all browsers. Anyone any ideas what the problem might be?

Thanks.

Sal.