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
-snip-
talking about spares...
spare riser, space cable, spare battery. if you troubleshoot and confirmed that a certain "stuff" is dead/with issues/problematic. throw them away, why? because in the future when you troubleshoot again, you might think that "dead/with issues/problematic" stuff is a "spare" LOL at a certain future headache hehe