If you had a chance to review the thread I posted earlier, you probably saw that all other forum members who experienced this symptom found underlying problem in their infrastructure/network. Whether it was an IP resolution, routing, etc.
I recommend that you review the logs one more time, then follow up with :
showmount -e [nasip]
rpcinfo [nasip]
basic network troubleshooting: ping, traceroute, MTU, etc.
Proxmox is using Linux Kernel and standard Linux NFS tools to query/talk to NFS, it would be very unusual for Linux to get into a situation where NFS issue is resolved by rebooting a host.
Good luck
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