You could ask the creator of the app here: “Sente - Online GO” App for Android
But my guess is that the app worked fine and your opponent marked your group as dead, and you clicked the “agree” button.
After both players have passed, there is a phase of the game where the players have to agree which groups are alive and which groups are dead. If they agree, the game is scored. If they don’t agree, then you have to resume playing to finish killing the groups that you think are dead.
If your opponent marks your group as dead and you think that your group isn’t dead, then you should not click the “agree” button!
This disagreement could be due to a genuine go mistake, or to a missclick, or in the worse case to sneaky behaviour by your opponent.