I wish we knew exactly.
There’s a hidden input field on the “Account Creation” screen that is checked for integrity at account creation time.
If this field is tampered with, the account creation fails.
Can you please ask your users to disable their plugins one-by-one until they locate the culprit?
We also have a ticket open with the Chrome project to investigate this behaviour.
Let me remind you of this screenshot from a non-technical user’s computer:
You can’t assume anything 