Adding two XML block attributes in different categories in Internet Explorer 11 results in broken second Online Editor.

Steps to reproduce
1. In ezpublish_legacy/settings/override/content.ini.append.php add two new categories (at the end of the file):

[ClassAttributeSettings]
CategoryList[test1]=Test1
CategoryList[test2]=Test2

2. In legacy backoffice create new class Test Class with attributes:

3. In Internet Explorer 11 create new content of class Test Class

Result
There will be javascript error in console Unspecified error and second Online Editor will not render (see attached screenshot).