calabash-android: common problems with solutions (screenshots attached)

1) java.lang.SecurityException: Permission Denial: starting instrumentation ComponentInfo{com.appname.test/sh.calaba.instrumentationbackend.ClearAppData} from pid=2904, uid=2904 not allowed because package com.appname.test does not have a signature matching the target com.appname.

this is seen when you are trying to run tests against app which was built on different machine but tests are run from your machine. In this case application built on different machine will have different signature (incase you haven't shared the keystore file with them) and your test server cannot communicate with the app

Solution: 
manually uninstall app from device
take the apk file and resign it - calabash-android resign app.apk
now run tests , they will work fine

2) invalid multibyte char (US-ASCII)
Add below on top of your ruby source files.

# encoding: utf-8