Adding CIFS storage from GUI fails with error message:

create storage failed: error during cfs-locked 'file-storage_cfg' operation: mount error: Refer to the mount.cifs(8) manual page (e.g. man mount.cifs) (500)

We will add share via CLI, because you need to include smbversion 2.1:

pvesm add cifs <PROXMOX_NEW_VOLUME_NAME> --server <QNAP_IP> --share <SHARED_FOLDER_ON_QNAP> --username <QNAP_CIFS_SHARE_USERNAME> --password <QNAP_CIFS_SHARE_PASSWORD> --smbversion 2.1


pvesm add cifs QNAP --server --share Proxmox --username proxmox --password proxmox.pass --smbversion 2.1

Share is now mounted.

Go to the GUI:

Datacenter -> Storage -> QNAP -> Edit -> Content -> Select the content you want and click OK.