The issue is not on your end. I was able to reproduce the same issue and have reported it to the development team that maintains that add-on.
In the meantime it should work using locally attached storage (if you plug in a drive it should show up in Kodi browsing root filesystem>media), a system mount pointing to your network location [more information], or using the “Backup” add-on found in Kodi’s repository as an alternative to OSMC bespoke utility.