There is a known bug (EXTPLESK-5497) when are you using the build-in monitoring and disable the monitoring for a particular Disk (for eaxmple /boot/efi) via Settings > Thresholds, the Disk dashboard shows "Dashboard Not Found". In most cases this can be solved by enabling monitoring for each disk (Settings > Thresholds). Otherwise re-installing both the Monitor and Grafana extension should solve this too.