All the error tells is that the file isn't valid XML (at least not at the end, there is some valid data first); it's impossible to tell exactly what can be done. If it doesn't contain sensitive information you can send it to me privately so I can take a look (fraca7 AT free DOT fr).

As for the cause, the only one I can think of would be a hard reboot, power outage or such. The task file can't get corrupted at the application level, even if the disk is full.