I've just downloaded and installed the latest asset versions for Raspberry Pi and Debian Linux and found that a new directory has appeared in the installation: "Asset-R6-RaspberryPi-registered". This no longer matches the recommended path set in the "Setting Asset to start automatically" instructions which has "@reboot /usr/bin/asset/bin/AssetUPnP".
To fix this for now, I've moved the bin directory up one level from "/usr/bin/asset/Asset-R6-RaspberryPi-registered/bin" to "/usr/bin/asset/bin", rebooted and all is well again.