Cannot add blocks: ResponseText: The website encountered an unexpected error. Try again later.TypeError: call_user_func_array(): Argument #1 ($callback) must be a valid callback

Problem/Motivation

When adding a new block to the page Ajax exits with a 500 error and

"ResponseText: The website encountered an unexpected error. Try again later.TypeError: call_user_func_array(): Argument #1 ($callback) must be a valid callback"

Steps to reproduce

Try to add a block.