It sounds like that app is wreaking havoc on your phone. Try starting the phone in "Safe Mode" and see if you can then uninstall the app. It should let you. Then restart the phone and see if the issue is resolved.
To get into "Safe Mode" please do the following.
1. First turn off the phone.
2. Next press and hold "Power" until you see the manufacturers logo on the screen, then release the power button.
3. "Quickly" press "Volume Down" and hold it for few seconds until you see the message "Safe mode: ON" or something similar.
4. If you wanted disable it, simply repeat the steps above, and you should see "Safe Mode: OFF" rather than on.