Unfortunately, trying to upgrade ownCloud on an Asustor AS-602T to version 6.0.3 running Asustor Data Master (ADM) 2.1.1.R3B2 resulted in a corrupted installation. Even removing and re-installing ownCloud via the App Central didn’t solve this issue.
So, in order to solve this issue and remove your corrupted ownCloud installation from your system first remove it via the App Central and then SSH to your NAS and remove the following folder:
NOTE: The following command will remove your installation and settings!
root@NAS:/volume1/.@plugins/AppCentral # rm -rf owncloud/
This fixed the ownCloud upgrade problem and I was able to install ownCloud 6.0.3.