Ext. FTP not accessible from mobile client

I have connected an external FTP site to our Next 27.01
This works perfect in browser and also in local windows client.

Mobile client (Android) can access all, but it can not access the FTP. It gives no error, it simply says ‘loading’ and after few minutes it gives up and nothing is displayed.
Is this somehting known? Or can I do something to display and access the contents of the external FTP?

No known issues that I’m aware of, but I’m not sure how heavily FTP is used with External Storage.

What do the NC server-side log entries associated with the Android client access attempts look like?