I am using IE 11 and I had the same issue and that was to do with the Compatibility Settings. Removed the compatibility settings and I was able to click Ok and was able to save the value and close the window. Can you check and see whether that helps. Thanks
Unfortunately this is not a solution for me.
I am using IE 11, removed the compatibility settings, restarted IE.
But the error message still comes up, as described in the first message.
Just for information: currently we are using Nintex Workflow 2010.
Any idea is much appreciated.