Thread:Extension talk:ParserFunctions/Fatal exception of type MWException Error/reply (31)

I know this thread is a little old, but I just had this error upgrading from 1.22.8 to 1.23.1, and I thought I'd provide the solution that helped me.

In my case, I had a poorly-coded skin that seemed to work on 1.22.8. I guess it used deprecated functions or something, and didn't work caused the Fatal Error when I accessed Special:Preferences.

I changed the default skin to Vector, and all is well in the world.