Contact form doesn't work

I have a contact form on my website from Adobe Muse. It was working fine, but it stopped, not exatcly sure when. When I try to send a message from the form it says that "the server encountered an error", and that's it. I tried different email accounts (ovh-based and not) and still the same.
You can also run a diagnostics:
http://burzynski-translations.pl/scripts/form_check.php
and it doesn't return any php errors from the host.
Muse is not supported anymore, so I don't think anything changed there. What can I do make it work?

**Requirements to fill in:**
- Domain name : burzynski-translations.pl
- I have standard hosting, the most basic version.



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