Hi Damiano,
I have checked the plugin and I have got the same error when I try to save the options in that page. I thought some plugin or the main theme were incompatble with the plugin, but they are not. I don’t know what happens.
We can try two possible solutions:
1.- Set the date in the database through phpmyadmin.
2.- Uninstall the plugin, and install it with the date you want by default.
Regards