TypeError: xhr.responseJSON is null

Hello,
I use WordPress, CF7 and Conditional Fields for Contact Form 7 in the latest version. Unfortunately, when I submit my form, I sometimes get the error message in the JavaScript console:

Failed to load resource: the server responded with a status of 500 (Internal Server Error)
scripts.js?ver=1.3.4:241 Uncaught TypeError: Cannot read property 'mailSent' of null
    at HTMLDocument.<anonymous> (scripts.js?ver=1.3.4:241)
    at HTMLDocument.dispatch (jquery.js?ver=1.12.4:3)
    at HTMLDocument.r.handle (jquery.js?ver=1.12.4:3)
    at Object.trigger (jquery.js?ver=1.12.4:3)
    at Object.a.event.trigger (jquery-migrate.min.js?ver=1.4.1:2)
    at y (jquery.js?ver=1.12.4:4)
    at XMLHttpRequest.c (jquery.js?ver=1.12.4:4)

What can be the reason? Many thanks for your help!

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