We've looked into this:
IE11 issue:
The version of TinyMCE used in the visit editor has a couple of incompability issues with IE11. We're in the progress of upgrading to TinyMCE 4, so we won't spend any time on upgrading the existing version to a newer 3.x version until then.
As a temporary fix, you can make it work by adding a meta tag to the page:
<meta http-equiv="X-UA-Compatible" content="IE=EmulateIE10">
This makes IE11 emulate IE10, and makes IE11 work with the editor. Not an ideal fix, but the real fix will come shortly with TinyMCE 4.
Custom editorsettings
The reason for the error message is that the wafinsertimage plugin is loaded, but not the button. This causes some of the code to be loaded, but not all.
The fix:
protected override void OnInit(EventArgs e) {
EditorSettings settings = new EditorSettings();
settings.AddSetting("theme_advanced_buttons1", "bold,italic,underline,separator,bullist,numlist,undo,redo");
settings.AddSetting("plugins", "inlinepopups");
editorContent.EditorSettings = settings;
base.OnInit(e);
}