Theme – Customization Page not loading

Hey Guys,

I updated our Parent-Theme and I’m running into bags of errors. I fixed many of them, but there is one that I can’t get rid of:

Before the update I couldn’t customize the theme, because of that “cheatin. uh?” error. I like how it tells you what’s going wrong. However, I’m running a multisite installatio, with 2 Child Themes. One for production, one for testing. At this time they are the same, but the test one is running fine, while the production one has bugs. Liek I said, I couldn’t edit the theme earlier. After the update it’s nearly the same situation: test runs fine. But production isn’t even able to show me the whole customization page now. It shows “Active Theme” in the left bar and the loading circle in the right area. But that’s all. Nothing happens.

I can’t find any help and my brain hurts. I don’t know what to do…Help me