@vmeyer, how old is this SIAC or when was the last battery exchange?
From what you describe, it may be one of a very small number of SIACs with an older firmware that had a bug whereby in some weird situations the SIAC "locked up" when the battery died. See https://www.sportident.com/news/350-siac-service-n...
for the news blog on this and https://www.sportident.com/documents/cards/sportid...
regarding the bug pre-fw4.0.
Honestly it's hard to believe the SIAC battery could have survived this long (over 7 years), but your discription does fit.
There are some really hairy (and partially destructive) steps to rescue the chip data if needed, otherwise a standard battery exchange service will "unlock" the SIAC.
Feel free to contact me if you need further help (mail in my user profile).