Thanks for this Thread.
I was able to solve our Problem with this Thread, i had to replace the original Repo from CEPH with the Proxmox Repo, after "apt upgrade" i was able to mount cephFS from our external Server.
A little Background Info about our current Setup:
Currently we using an external CEPH Storage on Debian which already running on Mimic (deployed with Croit), so i have changed the repo to original from CEPH to get the Mimic Updates. Our external Backup CEPH is running with Proxmox itself, currently they is exporting the RBD through an Gateway NFS VM, but i want to use cephFS to storing the Backups on them. With the original Update from ceph we wasnt able to mount it. The Error Message: "create storage failed: error with cfs lock 'file-storage_cfg': mount error: dmesg | tail or so. (500)".
But now its solved.