@Michel_Vermeulen The issue in the setup you are describing is that one server creates the files in the media folder but another may be the one that will serve the media files depending on how the load balancer selects its backends. That will indeed not work with Kirby without modifications.
An alternative (which we use for the Kirby site and also our Kirby Demo) would be to use a pull-zone CDN for all media files that points to the media directory of one of your servers. You can then pre-generate all media files like we do it with the Kirby Demo, so that they are available for the CDN. If you use a CDN that has thumb capabilities (like KeyCDN, which we use), you can avoid pre-generating all thumbs on the server.
If you want a simple solution, you can configure your load balancer to always direct the requests of a specific user to the same backend. Then each backend can manage its own media folder and there won’t be issues like those you are describing.
BTW, what is the reason for the load balancer in your particular setup or which benefit do you see?