I have got the same problem . After ionic cli update from 4.2.0 to 4.2.1 . When i build app on device is not opening. When i install clean project after build app work after that i install some plugin ( from native plugin list lika admob or social sharing ) after build app crash on start. There is the way to debbug why app is crashing ?
Did you add or update any package since the old version?
I had this kind of issue in the past on a dependence conflict in / platform / android / project.properties.
2 plugin asked for 2 different version of a library and the older version was in the project.properties
An other time the project.properties included the same library 2 times with differents version.
Did you try to remove platforms folders and reinstall them.
The adb logcat command should show you the java exception which cause the issue.
With no more informations or logs…
An other way to find where it come from is to start a blank project and add a group of dependancies and test group by group. After configs and dependancies added if the issue is not in the dependancies try to add your code again group by group.