"Insert Symbol" popup does not load using IE 11

Steps to reproduce:

1. Go to the editpage.action from any page
2. Click the "+" (Insert more content) icon, and select Symbol
3. The Insert Symbol window will be displayed, but no symbols come up inside the window.

Other browsers, like Chrome and Firefox works fine

Customer is running Confluence 5.6.3. I were able to reproduce it in two local environments running Confluence 5.8.5 and also IE 11.

Please refer to the screenshot below for more details: