I upgraded Owncloud from version 7 to version 8 using the Debian package repos. In the process, something must have gone wrong because the website now showed just a blank page. Curiously, the app continued to work.

Nothing showed up in the Apache server logs, nor anywhere else that I could find. Finally I opened up index.php and started tracing through the code. Turns out I was getting an exception with a description of

js file not found: script:js/chosen/chosen.jquery.min

I found this post which had the solution. https://github.com/owncloud/core/issues/13999

Just as the OP did, I had files_mv enabled. I had a bunch of others, but disabling that one was all it took to get the website working again.