after restarting the plex lxc the content previously mounted with smb is no longer available!?

[hpoperator]

hello dear community,

am currently facing a problem regarding the setup around Plex lxc.
I get everything set up so far. Everything is up and running.
I now point in CLI with smb to the media content that is on my disk storage that I passed on openmediavault, set the permissions and configuration file .mount, also start the directory service to automatically mount the previously created folder on restart, but after restarting the LXC the folder that I previously mounted in pex with smb is gone again.
So again the same game.... with the same result. It runs until I restart the lxc.

For the purpose of mental reconstruction here is my way:

apt install cifs-utils -y

nano /root/.cifs_credentials

#username=ABCDEFGH
#password=VWXYZ

(my SMB credentials)
then ctrl+O ....strg+X

chmod 600 /root/.cifs_credentials

mkdir /mnt/plex

id -u plex && id -g plex
(all as you wish)

nano /etc/system/system/mnt-plex.mount
Contents:

[Unit]
Description=Plex Media Server files
Requires=network-online.target
After=network-online.service

[Mount]
What=//192.168.178.40/MY_MEDIA
Where=/mnt/plex
Options=credentials=/root/.cifs_credentials,uid=<uid>,gid=<gid>
Type=cifs

[Install]
WantedBy=multi-user.target

systemctl enable mnt-plex.mount

systemctl start mnt-plex.mount

ls -l /mnt/plex
All my media folders are listed!

The media content was also mounted correctly and I can watch all media content via plex..... wonderful!

After a reboot=
I can't open any media and there is no content in mnt/plex either


What can I do?
What am I doing wrong?

I would be very grateful if you could help me. I would like the media content to be mounted/still available after the LXC reboot.

(Postscript:
the OMV-VM, which provides the disk storage, is also restarted and the disks, which I also mounted with SMB in Windows, are also all properly accessible)

Thanks a lot!