The system is designed to be running under an UPS. Under some circumstances, an unexpected power failure may corrupt some files that are critical for the system. This is true for any computer system. Check if this applies to you by restarting mosquitto, for example like this (may differ from platform to platform).
The error message implies that there is a database corruption and you need to delete the database file of mosquitto. It is ussually found in
$SCPATH$ is the path where you have installed Site Controller. In case it is not there, you will need to read your mosquitto configuration path and see where the database file for mosquitto is stored.