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Oops! 

It's "token verification failed" in my case.

Token failed is "kind" of normal in certain conditions.
The tokens are a security feature that (simplifying) allows to submit a form only once, that way, if you are tricked to submit data of a form by an hacker or so, it's very difficult to get the token right, and most likely you'll face the token verification failed.

Unfortunately, you'll get the same error if for example you submit a form, receive an error and you use the "back" button of the browser to go back, because what happens is:
Elk generates a token and adds it to the form
you submit the form
first thing that Elk does is check if the token is correct, if it is, the token is invalidated and the program proceeds
Elk generates an error maybe because you set something wrong and gives you the "fatal error page" with the "back" button
if you use the browser's back button, you will come back to the original form, but this form contains the token that has been invalidated
you re-submit the form
* Elk verifies the token, but it finds it was invalidated and gives you the "token verification failed" error.