I'll weigh in on this. Have 3 androids, 2 are having same problem. None have ever been rooted.
AT&T Note 5 (no external SD card), Android 5.1.1 is not affected.
AT&T Galaxy S5 Active and WiFi Samsung TabPro 12.2, both running Android 5.1.1, both with SD card and apps moved to SD are having this problem.
Automatic updates are not installing from the Google Play store. No other stores have been accessed except Samsung direct downloads from the Galaxy Apps app.
Google Play indicates updates to be installed. Go to the Play store, click "Update all" and click the appropriate access approved for the apps that need approvals. Download begins. Download finishes and restarts, sometimes several times. Eventually finishes downloading and starts installing. Never finishes install, eventually gives Error 963. Have followed the store's instructions for troubleshooting the error with no luck. Only thing that has worked so far has been to uninstall the app needing updating and reinstall it. Have cleared caches and data on Play store and download manager, have rebooted WIFI router. This happens with wifi and 4gLTE both.
Any suggestions other than a hard reset to factory defaults?