Error message on account creation

There is a strange error message on my live app when a user creates an account:

As my app is in French and the error message too, I’ve looked it up in the Settings> Languages, and here is the original message in English: “CORE - When the user tries send an email without a subject line”.
Problem: I have no idea what this is about!
Any clue, someone?
Thanks in advance.

Ok, I think I’ve found the solution: this was due to the workflow when creating a account that was trying to send an email to the admin without a subject…