Contactform 7 have tp press submit twice to work

Hi,

we use a standard Contactform 7 form with fields Businessname, name, email, phone and comment.
When clicking on submit, we get the error message with the orange border.

When we click submit for a 2nd time (with field content unchanged as filled-in in the first try), it works and the message is send.
But we do not use Akismet or Recaptcha antispam.

Anyone any idea what causes this or how we can diagnose whats wrong ?

Thanks !

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