Don't source plugins from PGB -- they are old and outdated. Use NPM instead: phonegap-plugin-push
Furthermore, SafariViewController should not be interfering in any way, since it isn't supported on Android (it's iOS only).
Also, make sure to pin your app to a specific version of PGB. See: Adobe PhoneGap Build
Thank you so much for your reply, Kerri. You gave me some nice insights for what and where I had to investigate.
After that I was able to learn a lot about the best practices for phonegap and ended pinning every plugin for specific versions.
It appears that these settings won't build because SafariViewController was having some conflicts with CustomTabs stuff.
<gap:plugin name="cordova-plugin-safariviewcontroller" source="npm" />
<plugin name="com.phonegap.plugins.pushplugin" spec="2.5.0" source="pgb" />
So I changed this for this and everything worked like a charm.
<plugin name="cordova-plugin-push-notification" spec="2.5.2" />
<plugin name="cordova-plugin-safariviewcontroller" spec="1.4.1" />
The next steps will be to update the code from the old PushPlugin to the new one and to update the Phonegap version.