Problems with NUT

I recently replaced my UPS and set up NUT (Network UPS Tools) for monitoring.

Everything works fine for a short time, but after a few hours, NUT reports "error: Data stale" in the web UI. After a few more hours, the web UI just stops responding completely.

I have NUT running on a Ubuntu VM in Proxmox. My UPS is a Tripp Lite (Eaton) SU1000RTXL2UA. It's connected to my hypervisor via USB and passed through to the VM. Restarting the container fixes the problem. Any idea what could be causing this?