Hey @Dr.Axelbauer welcome back 
You posted your request in the support category. Unfortunately you ignored the template and a lot of information to help you is missing. Please edit your original post and add all required details like Nextcloud version, webserver type and version, os version, related log file content. Use the support template.
Without additional information the community members cannot help you.
meanwhile:
advisable to use AppApi (External Apps) and Docker within my nextcloud lxd-container
there are some caveats when running Docker inside LXC, see wiki-md/LXD-LXC.Docker_in_LXC.md at scubamuc-wiki · scubamuc/wiki-md · GitHub, but if you keep that in mind, running Docker and your Nextcloud in the same LXC should work. but recent apparmor updates have been known to cause issues with security nesting preventing Docker inside LXC from accessing LXD host root binaries with required permissions! so you may want to consider LXC VM instead.
recommend creating separate LXC VM’s for Docker, thus keeping Nextcloud instance and Docker instance separate allowing easier troubleshooting:
LXD-Server example Docker LXC VM’s:
