oC 9.1.4 adding SFTP external storage doesn't work

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. Log in as yourself, Browse to "Personal" settings, scroll down to external storage
2. Select Type "SFTP" and enter the credentials
3. Click away from the text box and it will "test" the connection, showing a spinning circle icon
4. The spinning circle icon spins forever and never becomes a green "success" circle.
5. The SFTP server shows a successful login in, using both username/password and RSA shared key, but oC does not indicate this.

From the SFTP server log, showing successful login for both username/password and RSA shared key:
Feb 14 22:48:38 nostromo sshd[10679]: Accepted password for george from port 42940 ssh2
Feb 14 22:48:38 nostromo sshd[10679]: pam_unix(sshd:session): session opened for user george by (uid=0)
Feb 14 22:55:55 nostromo sshd[13676]: Accepted publickey for george from port 42948 ssh2: RSA e7:a0:d3:a6:0b:bb:f1:f9:21:f4:2b:65:f1:47:26:74
Feb 14 22:55:55 nostromo sshd[13676]: pam_unix(sshd:session): session opened for user george by (uid=0)

Expected behaviour
The spinning circle should become a green success circle after the login succeeds.

Actual behaviour
The spinning circle spins forever, and browsing to oC "Files" also shows a forever spinning circle. The only way to restore normal functionality is to delete the SFTP external storage.

Server configuration
Operating system: CentOS 7.3
Web server: Apache 2.4
Database: MariaDB 5.5.52
PHP version: 5.5.21
ownCloud version (see ownCloud admin page): 9.1.4
Updated from an older ownCloud or fresh install: updated from 9.1.3
Special configuration (external storage, external authentication, reverse proxy, server-side-encryption):

ownCloud log (data/owncloud.log)

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

There are no new lines written to owncloud.log during this activity.

Integrity status for oC9+

Login as admin user into your ownCloud and access
paste the results here.
No errors have been found.

Without matching logfiles its probably not possible to help here. You might want to create a bugreport to the developers about that at https://github.com/owncloud/core/issues