Hmm, the error didn't repeat itself. I didn't save before the battle with the last demon, so I had to replay from a bit earlier. True, the scenario unfolded slightly differently – when the error occurred, I was defending in the castle, and he attacked me, but the second time, I was already besieging his castle. In my opinion, that's the difference – when I attacked, I was then transported to the top after the cutscene, and it took away the artifacts and the remaining movement points. And only the next day did it take away the skills. The first time, it took everything at once – maybe that's where the script glitched.