Email Form not working, cache problem?

Hello,
I'm using the form/email plugin but it is not working correct. After deleting the cache everything is alright for a few times, but after a while when i press submit i dont get the "thankyou"-Page. Its a Onepager (Basis: Landio). The contact form frontmatter looks like this

title: contact
hidemenu: false
cache_enabled: false

Do you have a suggestions, how to solve this. Thanks!

Are you all up to date on your plugins/admin/grav updates? What does the full page .md file look like?