WebDAV no longer working after upgrade to 9.1.2

Please help us by providing the following info. Before posting please also check the pinned "Known issues" threads and make sure that you're running the latest available version for your oC release: https://owncloud.org/changelog/

Steps to reproduce
1. Upgrade to 9.1.2 from 9.1.0

Expected behaviour
Things should continue working

Actual behaviour
Your web server is not yet set up properly to allow file synchronization because the WebDAV interface seems to be broken.

Server configuration
Operating system: Ubuntu Xenial (16.04 LTS)
Web server: nginx
Database: MySQL 5.7
PHP version: PHP 5.6
ownCloud version (see ownCloud admin page): 9.1.2
Updated from an older ownCloud or fresh install: Update
Special configuration (external storage, external authentication, reverse proxy, server-side-encryption):

ownCloud log (data/owncloud.log)
Empty!

Please paste possible errors in the following code block, see https://central.owncloud.org/t/how-to-find-webserver-or-oc-logfile-enable-php-logfile/808 for more info

No PHP Errors

Integrity status for oC9+

No errors have been found.

I've been troubleshooting this since yesterday, updated nginx to the latest version, updated PHP to the latest version, no change. There is no errors anywhere to be found.

PROPOUND returns 200
74.69.82.206 - admin [29/Nov/2016:12:42:00 -0500] "PROPFIND /remote.php/webdav/ HTTP/1.1" 200 386 "-" "Mozilla/5.0 (Macintosh) mirall/2.2.4 (build 3709)"

This is the result of PROPFIND in the browser:

<?xml version="1.0"?>
<d:multistatus xmlns:d="DAV:" xmlns:s="http://sabredav.org/ns" xmlns:oc="http://owncloud.org/ns">
 <d:response>
  <d:href>/remote.php/webdav/</d:href>
  <d:propstat>
   <d:prop>
    <d:getlastmodified>Mon, 28 Nov 2016 20:41:17 GMT</d:getlastmodified>
    <d:getetag>&quot;583c966e0057c&quot;</d:getetag>
    <d:resourcetype>
     <d:collection/>
    </d:resourcetype>
    <oc:fileid>2</oc:fileid>
    <oc:permissions>RDNVCK</oc:permissions>
    <oc:size>1377307263</oc:size>
    <oc:tags/>
    <oc:favorite></oc:favorite>
    <oc:comments-unread>0</oc:comments-unread>
    <oc:owner-display-name>admin</oc:owner-display-name>
    <oc:share-types/>
   </d:prop>
   <d:status>HTTP/1.1 200 OK</d:status>
  </d:propstat>
  <d:propstat>
   <d:prop>
    <d:getcontenttype/>
    <d:getcontentlength/>
   </d:prop>
   <d:status>HTTP/1.1 404 Not Found</d:status>
  </d:propstat>
 </d:response>
 <d:response>
  <d:href>/remote.php/webdav/DigitalHDA/</d:href>
  <d:propstat>
   <d:prop>
    <d:getlastmodified>Mon, 28 Nov 2016 20:41:17 GMT</d:getlastmodified>
    <d:getetag>&quot;583c966e0057c&quot;</d:getetag>
    <d:resourcetype>
     <d:collection/>
    </d:resourcetype>
    <oc:fileid>40</oc:fileid>
    <oc:permissions>RDNVCK</oc:permissions>
    <oc:size>1372553049</oc:size>
    <oc:tags/>
    <oc:favorite></oc:favorite>
    <oc:comments-unread>0</oc:comments-unread>
    <oc:owner-display-name>admin</oc:owner-display-name>
    <oc:share-types/>
   </d:prop>
   <d:status>HTTP/1.1 200 OK</d:status>
  </d:propstat>
  <d:propstat>
   <d:prop>
    <d:getcontenttype/>
    <d:getcontentlength/>
   </d:prop>
   <d:status>HTTP/1.1 404 Not Found</d:status>
  </d:propstat>
 </d:response>
 <d:response>
  <d:href>/remote.php/webdav/Documents/</d:href>
  <d:propstat>
   <d:prop>
    <d:getlastmodified>Mon, 24 Oct 2016 16:44:21 GMT</d:getlastmodified>
    <d:getetag>&quot;580e3a65b7116&quot;</d:getetag>
    <d:resourcetype>
     <d:collection/>
    </d:resourcetype>
    <oc:fileid>3</oc:fileid>
    <oc:permissions>RDNVCK</oc:permissions>
    <oc:size>36227</oc:size>
    <oc:tags/>
    <oc:favorite></oc:favorite>
    <oc:comments-unread>0</oc:comments-unread>
    <oc:owner-display-name>admin</oc:owner-display-name>
    <oc:share-types/>
   </d:prop>
   <d:status>HTTP/1.1 200 OK</d:status>
  </d:propstat>
  <d:propstat>
   <d:prop>
    <d:getcontenttype/>
    <d:getcontentlength/>
   </d:prop>
   <d:status>HTTP/1.1 404 Not Found</d:status>
  </d:propstat>
 </d:response>
 <d:response>
  <d:href>/remote.php/webdav/Photos/</d:href>
  <d:propstat>
   <d:prop>
    <d:getlastmodified>Mon, 24 Oct 2016 16:44:22 GMT</d:getlastmodified>
    <d:getetag>&quot;580e3a662527b&quot;</d:getetag>
    <d:resourcetype>
     <d:collection/>
    </d:resourcetype>
    <oc:fileid>5</oc:fileid>
    <oc:permissions>RDNVCK</oc:permissions>
    <oc:size>678556</oc:size>
    <oc:tags/>
    <oc:favorite></oc:favorite>
    <oc:comments-unread>0</oc:comments-unread>
    <oc:owner-display-name>admin</oc:owner-display-name>
    <oc:share-types/>
   </d:prop>
   <d:status>HTTP/1.1 200 OK</d:status>
  </d:propstat>
  <d:propstat>
   <d:prop>
    <d:getcontenttype/>
    <d:getcontentlength/>
   </d:prop>
   <d:status>HTTP/1.1 404 Not Found</d:status>
  </d:propstat>
 </d:response>
 <d:response>
  <d:href>/remote.php/webdav/ownCloud%20Manual.pdf</d:href>
  <d:propstat>
   <d:prop>
    <d:getlastmodified>Mon, 24 Oct 2016 16:44:22 GMT</d:getlastmodified>
    <d:getetag>&quot;90b7d244ab044565a7c47d6d9c43c916&quot;</d:getetag>
    <d:getcontenttype>application/pdf</d:getcontenttype>
    <d:resourcetype/>
    <oc:fileid>9</oc:fileid>
    <oc:permissions>RDNVW</oc:permissions>
    <d:getcontentlength>4039431</d:getcontentlength>
    <oc:tags/>
    <oc:favorite></oc:favorite>
    <oc:comments-unread>0</oc:comments-unread>
    <oc:owner-display-name>admin</oc:owner-display-name>
    <oc:share-types/>
   </d:prop>
   <d:status>HTTP/1.1 200 OK</d:status>
  </d:propstat>
  <d:propstat>
   <d:prop>
    <oc:size/>
   </d:prop>
   <d:status>HTTP/1.1 404 Not Found</d:status>
  </d:propstat>
 </d:response>
</d:multistatus>

Which seems to indicate everything is okay? Except for the 404's, but I think that's normal too.
Using curl with a wrong username/password:

<?xml version="1.0" encoding="utf-8"?>
<d:error xmlns:d="DAV:" xmlns:s="http://sabredav.org/ns">
  <s:exception>Sabre\DAV\Exception\NotAuthenticated</s:exception>
  <s:message>Username or password was incorrect</s:message>
</d:error>

Using the right password with Curl:

curl --request PROPFIND  --user admin:EReS2fah9gaesaec --header "Content-Type: text/xml" --header "Brief:t" --data "<D:propfind xmlns:D='DAV:'><D:prop><D:displayname/></D:prop></D:propfind>" https://cloud.digitalhda.com/remote.php/webdav/
<?xml version="1.0"?>
<d:multistatus xmlns:d="DAV:" xmlns:s="http://sabredav.org/ns" xmlns:oc="http://owncloud.org/ns">
 <d:response>
  <d:href>/remote.php/webdav/</d:href>
  <d:propstat>
   <d:prop/>
   <d:status>HTTP/1.1 418 I'm a teapot</d:status>
  </d:propstat>
 </d:response>
 <d:response>
  <d:href>/remote.php/webdav/DigitalHDA/</d:href>
  <d:propstat>
   <d:prop/>
   <d:status>HTTP/1.1 418 I'm a teapot</d:status>
  </d:propstat>
 </d:response>
 <d:response>
  <d:href>/remote.php/webdav/Documents/</d:href>
  <d:propstat>
   <d:prop/>
   <d:status>HTTP/1.1 418 I'm a teapot</d:status>
  </d:propstat>
 </d:response>
 <d:response>
  <d:href>/remote.php/webdav/Photos/</d:href>
  <d:propstat>
   <d:prop/>
   <d:status>HTTP/1.1 418 I'm a teapot</d:status>
  </d:propstat>
 </d:response>
 <d:response>
  <d:href>/remote.php/webdav/ownCloud%20Manual.pdf</d:href>
  <d:propstat>
   <d:prop/>
   <d:status>HTTP/1.1 418 I'm a teapot</d:status>
  </d:propstat>
 </d:response>
</d:multistatus>

So it SEEMS the WebDav server is working, it lists the contents of my root folder yet no client wants to connect. The own cloud client says "invalid response to an authenticated webdav request"

Hi,

if something stopped to work after an minor update without any changes on the base infrastructure / environment please report that directly to the bugtracker at https://github.com/owncloud/core/issues