The exception occurs while processing the configuration file (config.txt). My best guess, without actually seeing the config file, is that your SDcard is worn out or otherwise damaged and the config file is compromised. Building a new SDcard from scratch may resolve the problem.
You might consider copying the history file initially to see if it may be undamaged and thus retain your history. (/iotawatt/histlog.log).
IoTaWatt does not maintain accurate file date/times on the SDcard. It doesn’t always know the date and time when creating files during startup.