There is an issue in line 830 of lib/webdavlib.php 

if (preg_match('#(application|text)/xml;\s?charset=[\'\"]?utf-8[\'\"]?#i', $response['header']['Content-Type'])) {

This is case sensitive. Headers should be case insensitive https://www.w3.org/Protocols/rfc2616/rfc2616-sec4.html#sec4.2 

I fixed the issue by putting the following if statement ahead of the problem one 

if (isset($response['header']['content-type']))

This was blocking me connecting moodle to an owncloud server