Creating a backup with adb results in 41 byte file
Found solution here, worked for me:

https://docs.j7k6.org/android-adb-backup-41-bytes-549-bytes-bug-fix/


  Appearantly the adb backup command stopped working for older Android
  devices in recent versions of the Android Debug Bridge. When invoking
  a backup with adb, the backup process is started but stops
  immediately, leaving a local backup.ab file with a filesize of only 49
  bytes (549 bytes when the backup is encrypted with a password). This
  bug is well known, but hasn’t been fixed at least until recently
  (version 1.0.39). To fix this behaviour, download the last working
  version of the adb executable (1.0.31) and invoke the backup with
  this. The executables for all platforms can be downloaded at Mozilla.


https://archive.mozilla.org/pub/labs/android-tools/

adb-1.0.31-linux.zip    479K    09-Sep-2015 18:44
adb-1.0.31-linux64.zip  455K    09-Sep-2015 18:44
adb-1.0.31-mac.zip  433K    09-Sep-2015 18:44
adb-1.0.31-windows.zip  474K    09-Sep-2015 18:44


Download, unzip, run unzipped adb:

(linux, mac)

./adb devices
./adb backup -apk -shared -all -f backup-file.adb


(win)

adb devices
adb backup -apk -shared -all -f backup-file.adb


Enjoy! ;)