Hi Gräzer Björn,
Thank you for reaching out, and I am sorry for the inconvenience. This issue might be related to your browser cache.
Please try the following steps:
Clear your browser cache by following this guide:
SP Page Builder Troubleshooting Guide.
Alternatively, you can follow this tutorial
If these steps do not resolve the issue, please enable Joomla's debug mode:
Error Reporting and Debug Mode Screenshot and give the error Screenshot.
Let me know if your issue is resolved or if further assistance is needed.
Best regards