Hi kamil.makowski88,
A likely reason for Push Notifications working without ProGuard, but not working with it is that ProGuard stripped out some of the classes/methods/fields needed for Push Notifications to work. This can occur if the aforementioned classes/methods/fields are invoked through reflection in Java.
To be more specific, ProGuard can handle stripping out unused classes/methods/fields for things that aren’t explicitly called, but as soon as you try to call something dynamically (via reflection), ProGuard won’t be able to know ahead of time to not strip out classes/methods/fields called this way.
If you know what these missing classes/methods/fields are, you can tell ProGuard to NOT strip them out using a proguard.cfg file.