Hey @johannes50! It should also work when using the plugin. Could you please try following the instructions in this article63 (first try the 'Refresh Forms' button, and then if that doesn't work, the API reset)?
Also, please try clearing any caching plugins on your website, if applicable!
If the issue persists after that, please reach out to our team at email@example.com (or via the in-app messenger in your account) with a description of this issue, the name of the landing page (as it is displayed on the forms page43 in your account), and the URL on which it is embedded. One of our Product Specialists will be able to help troubleshoot further! :)