Messages thru website are not being sent correctly

Hi there,

I am using fastest cache premium with with all options selected except the:
1. Logged-in Users;
2. Mobile;
3. Mobile theme.

namely 3rd; 4th and 5th options are not selected.

The reason why I am writing is that some clients started noticing us that they write us thru contact form and messages are not being sent.

Tried myself and got the message sent only per second time. First one, looked like the page was cached and nothing happened. The link is here: https://www.badetonneholz.de/kontaktieren-uns/

If sending was successful, you will see green message, saying “the message sent correctly” otherwise – the messages are not getting thru.

Tried to deactivate cache and messages were sent successfully, tried to delete all the cache including CSS/JS and messages were sent successfully as well. So the bottom line is cached files.

How to get this working? Thanks.