UCS in VM: smb-share not shown

I installed UCS 4.4+5.0 in VMs under PVE 6.4 (https://www.proxmox.com/en/proxmox-ve) on a host. I installed the App Nextcloud under UCS. I configured a smb-share.

I can see both the smb-share and the Nextcloud share from clients outside the host, but from within the host, from VM-clients, I can not see any smb-shares of VM-UCS and VM-UCS-Nextcloud.

To identify the “location” of the issue, I installed ovm5 (https://www.openmediavault.org/) on a VM and on a PC outside PVE and on each I configured a smb-share. These both I can see on inside VM-clients and from outside-client on separate PCs.

All firewalls are switched off. All VMs / PCs can ping each other successfully.

To me this looks like an UCS-issue.

Any hints or ideas, what / how I can dig into this issue? If not, I will install wireshark and compare the smb-net-traffic from UCS/Nextcloud to omv5.

Best regards, Peter