Android app can't connect to server

Nextcloud version (eg, 10.0.2): 12
Operating system and version (eg, Ubuntu 16.04): Ubuntu 17
Apache or nginx version (eg, Apache 2.4.25): 2.4
PHP version (eg, 5.6): 7
Is this the first time you’ve seen this error?: yes

My android app can’t connect to my server. I get a generic error 'could not connect to the server. I can log on to the server fine via browser using https.
Any thoughts on what could be wrong? The log shows nothing.

Can you reliably replicate it? (If so, please outline steps):

The issue you are facing:

The output of your Nextcloud log in Admin > Logging:

The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):

The output of your Apache/nginx/system log in /var/log/____:

Remember, this information may be requested if it isn’t supplied; for fastest response please provide as much as you can :heart: Feel free to use a pastebin service, otherwise log files can be indented with 4 spaces on each line to present them in a friendlier way on the forum.

cc @mario @tobiasKaminsky