FileReference.browse stopped working for 2 users ??

I have an application with approximately 400 users. File uploads are
regularly done using FileReference.browse by clicking a button.

This has suddenly stopped working for two users. When they click the Add
file button, the IE file browser just does not appear ???? Works fine for
everyone else.

Is there an obscure IE10 setting that may be preventing the file browser
from being displayed.

This is Flex 4.7
IE 10