the_block() is a custom function from a plugin or your current theme. If you google it, it seems to be related to a plugin called "Multiple content blocks" but I'm not sure (check to see if you have a plugin with that name, it might be disabled).
You are getting the error because there is a page on your theme trying to access a custom function which does not exist.
The fix I posted, creates a function the_block() thus eliminating the error. My fix simply outputs some invisible text whenever the theme tries to access the function.
Other than that, you'll need to contact a developer to take a deeper look.
Best of luck.