I want to make sure you'll get the help you need by routing you to the right support, userlea.
When it comes to integrating Datev and Elster into QuickBooks Online, it'd be best to contact their Customer Support since it's a third-party application. Any concerns about syncing or integrating the third-party software into your account are best handled by their technical support.
Just in case you need a third-party application to synchronize your Datev and Elster into QBO, just check out the tools available here at https://apps.intuit.com/app/apps/home. This is to ensure the application is integrated into QuickBooks. Intuit does not provide recommendations or support for third-party applications or websites; if you have an issue you will need to contact the provider of the third-party product.
Feel free to visit our Importing and Exporting Data page for more insights about bringing your data in and out in QBO.
I'm just one post away if you have other questions about preparing and managing your QuickBooks data. You have a good one.