Completely remove an application and its data on iOS 8 I have an application that is not working properly (I always get an error). I know other people who have the same app and for them no errors appears.

I thought that maybe there are temporary data stored and getting rid of them would solve the problem.

As suggested in this answer I tried to remove the app. I tried to delete it in two ways:
* By holding my finger on the icon and pressing the x.
* And from the menu Settings -> General -> Storage -> Manage Storage. 

None of them fixed the problem and I believe that some app data are not removed. I think so because when I remove the app and reinstall it, as soon as I open it I'm already logged in.

Is there a different way of getting rid of app data?
No, there is no other ways of erasing app data. As a developer you can't save data out of your bundle, and when you delete an app on your device, all bundle content is deleted.

Regarding the logged issue, one posible answer may be that the app is identifying your device and saving your configuration on internet. To ensure that is your case, you can delete and reinstall the app, turn off wifi and celular data just after install and without open the app. Then open up and check if your info is there.