Addmin new interface - protoclass.isCreateable is not a function

I have a UniFi AP AC PRO access point with OpenWRT installed on it. I need to configure access for admin and for guests, but I can't add a new interface using the Web-UI. Here you can see the console output when I click Create Interface:


It says protoclass.isCreateable is not a function. The process never finishes. What might be problem here?