Release version of the game +first hotfix. Bane spell adds +1d4 to all saving throws except for the Con save for wich it adds +1d20. It works the same in main campaign, user modules, and self made modules. It applies both to spells and scrolls. Other buff or debuffs doesnt seem to affect that.
I've found a thread 4 months old but no replies to it, it's a pity devs didn't address the issue for almost half a year(