I tried to put a spare battery on with no luck. But after posting this I've seen that not only the configs are not saved but the date/hour are also reset. So... maybe the spare battery is also dead. Ordered a new pack of batteries and it should arrive today. I'll try them and come back here to report.
Your spare battery is dead or expired, good brands hold the energy for some years, but cheap brands don't
About date and hour reseted, it's 99.99% sure it's battery, this is the exact sympton of dead battery, I'm sure you'll change and will works again
Follow the tip of user above, clean first and them put new battery
It's always good to have spare batteries if you have PCs, some programs can bug if your date is wrong
Last tip: if you have multimeter, you can test battery's life