Port 445 not visible in PBS (as virtual machine in PVE)

I have a fresh installation of PVE, in which I have a PBS virtual machine (I don't have a second hardware for a separate server).
-> I need to connect a disk to the PBS via SMB, which uses port 445.
Proxmox Backup Server
When I use "nmap -p 445 <IP>":

• PORT STATE SERVICE
• 445/tcp filtered microsoft-ds

Proxmox Virtual Enviroment
However, when I do the same on the host PVE, everything works:

• PORT STATE SERVICE
• 445/tcp open microsoft-ds

I have the port...​