Seems the following Wiki article has some additional info about oC9+ on QNAP:
https://wiki.qnap.com/wiki/OwnCloud#Owncloud_V9
However the statement there is not really true:
Owncloud 9 requires libxml version 2.7.0 or newer and that doesn’t seem to exist on any QNAPs at this time (June 2016).
The requirement was always there:
This has broken before as well but we just didn’t notice because peoples are not necessarily using our sync clients. Now with WebDAV in core also the web interface will break.
if just wasn’t enforced for versions below oC9: