The verification link can not be opened

I have set up the e-mail validation engine in line with Caldera’s instructions:
https://calderaforms.com/doc/verify-email-completing-caldera-forms-submission/

The mail also arrives at the person who completed and sent the contact form, but the link contained in it does not work!
What can I do?

Do I have to insert the validation code somewhere else?

Thanks in advance for your help!
Marie

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