I can upload ISOs to an NFS Datastore but not create VMs

[Nib51]

I'm new to Proxmox but finding it really intuitive to use. One thing that has stumped me though is that, while I can create an NFS Datastore through the Proxmox GUI on my QNAP NAS and it creates Ok and appears to be working (Proxmox successfully created folders on the NAS for ISOs, VM images etc.) and I can upload ISOs to the NFS datastore from within the Proxmox GUI, I cannot create a VM. I just get an error like this:

TASK ERROR: unable to create VM 100 - qemu-img: Could not open '/mnt/pve/ClusterStore/template/iso/name_of_iso.iso': Could not open '/mnt/pve/ClusterStore/template/iso/name_of_iso.iso': Input/output error

And if I try to clone from a VM Template I get an error like this:

TASK ERROR: clone failed: unable to create image: qemu-img: /mnt/pve/ClusterStore/images/108/vm-108-disk-0.qcow2: Could not open '/mnt/pve/ClusterStore/images/108/vm-108-disk-0.qcow2': Input/output error

This confuses me, because the fact that Proxmox could create folder on the NAS, and upload an ISO file to it implies that this is not an NFS permissions problem. Any advice?

Proxmox version = 9.0.3
QNAP NAS = TS-231
NFS version = 4

 

[Nib51]

I ended up finding that configuring NFS 3 (NOT 4) at both ends AND restarting BOTH the NAS and the Proxmox hosts fixed it.