I have the same error messages showing up on a web site I’m developing a plugin for. I do not have $acf in any of my code. It may be in one of the other plugins on the web site (it’s not mine to being making changes to).
However, I don’t think that “Do not use $acf for a variable name” is a fix.
The plugin developer should be using classes and namespaces to prevent other plugins from interfering with it’s operation.
