I had a constant problem with OC 8.2.7 and the Clients for OSX and Android. OSX Clients reported "Internal Server Error", and hung up the Syncs. After reading and hunting off and on for a solution for the last few months, I read that the problem maybe solved by an Upgrade o the Server to OC 9.1.1.
First I had to upgrade to OC 9.0.5 by unpacking the tarball (zip) and installing as directed by https://doc.owncloud.org/server/9.1/admin_manual/maintenance/upgrade.html.
It worked fine for upgrade from OC 8.2.7.
Upgrade to OC 9.1.1. produced errors, even if I followed exactly the same procedure as upgrading the OC 9.0.5.
Upgrade to the OC 9,1,1 produced an error at install and prompted to reload the browser page. Then the following appeared:
"This ownCloud instance is currently in maintenance mode, which may take a while.
This page will refresh itself when the ownCloud instance is available again.
Contact your system administrator if this message persists or appeared unexpectedly.
Thank you for your patience.
Now it hangs there and reloading the page makes no difference.
Checking with ps and top there does not seem to be any problems or overloads.