Contact form 7 field errors since start of 2018

General information for the website: A sports club website with news and ability to fill in a form join
Num. of web pages/modules: 1
Description of every page/module: Site already done, just a bug fix I cannot solve
www.berkshiretrisquad.com/join-bts
Description of requirements/features: Contact form. My form (join_bts) has worked fine for the last 2 years and somebody completed it without problem in December 2017. However, in 2018 the form will not submit. Each time it flags up and Error, saying

'that one for more fields are incorrect', however it does not say WHICH field is incorrect, it just won't work, very frustrating.

I just need [contact-form-7 id="4420" title="join bts"] to be working properly again.

I have already removed any on_submit code that I read about being an issue in 2018, but this has not made a difference.

Can you fix my problem and get the form submitting again??

Thanks
Paul
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