Hi there,
sorry to hear that. And I really don’t know at all, what to do with it. I don’t touch anything with the plugin. I just use the hooks for the rss_excerpt and the feed_content to modify the output by putting an image in front of it.
Usually the headers already sent stuff comes, when a plugin is causing an error and actually wants to write some error code, while wordpress is busy doing something else.
When testing the darned thing, I work in debug mode with error logs on, to really retrieve everything, I possibly could do wrong.
https://esc-blogg.atelier-fuenf.de/feed works perfect with a clean errorlog in a multisite installation.
https://wasistlos.waldemarstoffel.com/feed works perfect as a single site setup.
Do you have anything running on one of those websites, that might interfere? Like hosting your images on a cloud service?