Every time my phone reboots and sometimes even without reboot when it's working at I guess full capacity, it resets its settings and acts like I just booted the phone for the first time except from all the apps I've installed (i.e. non system apps) still being installed. However, I cannot open any non-preinstalled app as they just close immediately when I open them. Deleting the cache and/or force stopping the app doesn't help.
Since my last WhatsApp backup file msgstore.db.crypt12 is a couple days old, I tried to "rescue" the WhatsApp cache since the msgstore.db is just a backup file and WhatsApp stores the messages "live" in a different database that doesn't seem to be accessible without root (and I cannot root my phone at the moment).
The WhatsApp app-info shows that WhatsApp actually still occupies a lot of cache data and app data and the WhatsApp symbol on my application overview still shows the number of unread messages I had before the last unwanted reset.
I already tried backing up the whole app with as much app data as possible via adb with adb backup C:\wabackup.ab -apk -obb -shared com.whatsapp, uninstalling WhatsApp, reinstalling it and then restoring the backup but running the app afterwards just prompts me with the screen I would usually get when running the app for the first time after the installation.
The WhatsApp folder seems to be totally fine but maybe it's a permission issue with the folder. ( How to manually transfer entire Whatsapp installation to another device (the hard way) )
I would really like to fix this, not only for WhatsApp but also for other more important apps like banking-apps and 2-factor-authentication-apps that of course encounter the same problem at the moment but unfortunately I am not too familiar with the way Android stores app data.
This is pretty annoying obviously and I have already ordered a new phone. I am currently using the SM-J510FN.