I'm facing problem with login into sandbox with correct credentials.
My guess It's because com.clover.engine-2087.apk from(dev apks) works not with sandbox environment.
Steps to reproduce:
1. Create new Emulator
2. Download and setup com.clover.engine-2087.apk and com.clover.appupdater-1160.apk from https://sandbox.dev.clover.com/developers/dev-apks
3. Go to settings and add Clover account
4. Try to login with your Sandbox credentials. You will get error "Check your login credentials".
Also I've noticed that Engine app has title Clover(STG) and not Clover(DEV). Maybe this helps you to investigate the problem.