Everything is disappearing after click on the save button on contact form 7

Where can we see the website in question?

Dear Takayuki,

Thank you for your response. Here is the website link. https://sterlink.de/kontakt/
We are planning to release it on the 1st of December, so we put it on “coming soon mode”.

Thank you.

OK. What other plugins and theme do you use on the site?

I am using the following plugins and theme
1. Assistant
2. Beaver Builder Plugin (Pro Version)
3. Beaver Themer
4. SeedProd
5. Contact Form 7
6. GDPR Cookie Consent
7. Limit Login Attempts
8. myStickymenu
9. Swap Google Fonts Display
10. UpdraftPlus
11. Wordfence Security
12. Wp Headers And Footers
13. WP Mail SMTP Pro
14. WPS Cleaner
15. Yoast SEO

Try switching to the default theme and deactivating all plugins excluding Contact Form 7, Wordfence Security, and Yoast SEO.

Why you are advised to deactivate plugins and switch to the default theme.