Due to a permission check in \Craft\WebApp::processRequest(), I cannot access my plugin controller through the control panel templates. Also I cannot grant the user access because the plugin does not have a "cp section".
I have a fieldtype that loads data through an ajax-request, but this does not work if the user looking at the pages does not have access to the plugin.
Simple solution would be to always show all plugins (with controllers) in the permission overview, so that I can grant access if needed.