XF 1.4 The field was not recognised error message on registration form

No, that is unrelated as the errors are specifically related to the user name and email fields.

The easiest way to determine the cause is to test in a completely default unedited style with all add-ons disabled.

Then you can switch to the custom style and test again and re-enable add-ons one at a time to determine which one it is.
 
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