Can't log in to ownCloud from Android app, but web interface works

Actual behaviour

When I enter the server address in the official ownCloud app (the one that costs money on Google Play) and the ownCloud app tries to establish a connection before giving me the possibility to enter my login credentials, it says "Testar anslutning" followed by "Servern svarar inte." I don't know what the actual strings in English are, but they translate to "Checking the connection" and "The server doesn't respond."

If I try to connect to the server using HTTP, the second message is "Anslutningstiden till servern överskreds" ("Connection time to the server was exceeded") instead, after merely 3-5 seconds.

When I enter the exact same server address (copied and pasted) in a web browser on the same phone, I'm presented with a login page, and I'm able to login without any errors.

This is all done on my wife's phone. I have no trouble connecting from my own phone.

Expected behaviour

Log in successfully.

Steps to reproduce

  1. Open app for the first time
  2. Enter the address to the server (including https:// [yes, I've tried http as well])

Can this problem be reproduced with the official owncloud server?
(url: https://demo.owncloud.org, user: test, password: test)
No.

Environment data

Android version:
6.0.1
Device model:
Samsung Galaxy S5 SM-G900F
Stock or customized system:
Stock
ownCloud app version:
2.5.0
ownCloud server version:
10.0.4.4 (Community edition)

Logs

Web server error log

/var/log/apache2/error.log:

[Mon Jan 08 21:31:17.747365 2018] [authz_core:error] [pid 29589] [client 192.168.0.1:34344] AH01630: client denied by server configuration: /var/www/owncloud/data/htaccesstest.txt

#### ownCloud log (data/owncloud.log) No errors. Though I have some files that doesn't pass the integrity check:

- encryption
- EXTRA_FILE
- appinfo/Migrations/Version20170913113840.php
- lib/Command/RecreateMasterKey.php
- dav
- EXTRA_FILE
- appinfo/Migrations/Version20170927201245.php
- lib/DAV/MiscCustomPropertiesBackend.php
- lib/DAV/FileCustomPropertiesBackend.php
- lib/DAV/AbstractCustomPropertiesBackend.php
- lib/Connector/Sabre/CorsPlugin.php
- lib/Command/CleanupChunks.php