Hi, Meike 

It sounds this might be either a caching issue,

or perhaps a conflict with another plugin and Amelia...

Please deactivate Amelia, clear cache, activate Amelia, clear cache again. 

You can clear cache with the plugin such as "W3 Total Cache" or any other, also clear your browser cache and cookies.

if the issue persists, then try this :

Can you please change the theme and see if the issue is resolved? If not, then please deactivate all other plugins except for our plugin, refresh the page where the issue is, and see if it is resolved (it should be). If it is, then start activating other plugins one by one, and after each activation refresh the page to see if the issue reoccurred. Once it does reoccur, you will have found the plugin causing the issue.

If you configured everything, but the booking form doesn't work properly, it could be a cache issue. 

Let me know how it goes. Thank you