Update:
I got feedback from the Divi support. They provided:
1. A working temporary solution: In Theme Options / General /Custom css :
.admin-bar .et-fb-root-ancestor-sibling#fastspring {
display: none !important;
}
With this workaround the Divi frontend editor is no longer blocked while the FastSpring WordPress plugin is activated. Also now code snippets can be inserted in the text module for creating the FastSpring buttons on the page.
Using the FastSpring Buttons in the text module inspector does not work, because a click on these buttons opens a config window and when closing the window the buttons are not inserted.
2. For a final solution with a working configuration window with the Divi frontend editor, the Divi support told me, that also changes in the FastSpring WordPress plugin are necessary and that the Divi team would be glad to provide assistance for the plugin developers. They told me, that the Divi extension communication channel can be used for plugin compatibility issues by the plugin developers:
https://github.com/elegantthemes/create-divi-extension/issues
