Thank you for replying @sunjot16. I could solve the issue, it was a faulty overlay (but I cannot delete the question...)

We're running an instance on AEM 6.4.6.0. The authoring interface does not load anymore when attempting to author editable templates. It shows the following JS error: TypeError: ns.ContentFrame._EditableUtils is undefined Using the debugger, I can see that this function is affected, which seems to b...