When I want to log in with my iPad using the nextcloud app I always get an error 401 Unauthorized. The nextcloud app does not display the files. However, in the webinterface on my laptop I can see the nextcloud app on the iPad in the list of devices in the tab Security in the Settings.
Also logging in with the browser on the iPad works fine.
Nextcloud version: 19
iPad Pro 12.9 2018: iPadOS 13.5.1
I am happy for any help 
Please post your configuration on the ipad.
Do you use the correct nextcloud, username and password?
Can you access with the same app from iPhone or similar app from android with the same login credentials?
Do you use ssl-encryption with an correct certificate?
To exclude the nextcloud app and the iPad from the error you can test the app with another nextcloud e.g. https://try.nextcloud.com . If it works it is a problem of your nextcloud. If not it is a problem with your app and/or iPad.
If it works with Try-Nextcloud please use again your nextcloud and post logs from nextcloud and apache2 / nginx.
I tried try.nextcloud.com and got a Connection Error (404: The requested resource could not be found)
When I tried it on the laptop it worked.
error 404 is not normal. Have you register the user in the browser and then use in the the app https://demo1.nextcloud.com or https://demo2.nextcloud.com (url after registration)?
Example:
https://demo2.nextcloud.com
User: Wtc27QBbCcgKf8eQ
Password: demo
Now it worked. Sorry
Ok. There is no problem with iPad and your nextcloud ios-app.
Now test your nextcloud again an post logs from nextcloud and apache2/nginx.
I only found the following logs:
WarningcoreLogin failed: ‘Joel+Niklaus’ (Remote IP: ‘213.55.244.27’)2020-07-07T14:06:00+0200WarningcoreLogin failed: ‘Joel+Niklaus’ (Remote IP: ‘213.55.244.27’)2020-07-07T14:05:35+0200WarningcoreLogin failed: ‘Joel+Niklaus’ (Remote IP: ‘213.55.244.27’)2020-07-07T14:05:17+0200WarningcoreLogin failed: ‘Joel+Niklaus’ (Remote IP: ‘213.55.244.27’)2020-07-07T14:05:10+0200WarningcoreLogin failed: ‘Joel+Niklaus’ (Remote IP: ‘213.55.244.27’)2020-07-07T14:05:08+0200WarningcoreLogin failed: ‘Joel+Niklaus’ (Remote IP: ‘213.55.244.27’)2020-07-07T14:04:52+0200WarningcoreLogin failed: ‘Joel+Niklaus’ (Remote IP: ‘213.55.244.27’)2020-07-07T14:04:45+0200WarningcoreLogin failed: ‘Joel+Niklaus’ (Remote IP: ‘213.55.244.27’)2020-07-07T14:04:43+0200WarningcoreLogin failed: ‘Joel+Niklaus’ (Remote IP: ‘213.55.244.27’)2020-07-07T14:04:27+0200WarningcoreLogin failed: ‘Joel+Niklaus’ (Remote IP: ‘213.55.244.27’)2020-07-07T14:04:20+0200WarningcoreLogin failed: ‘Joel+Niklaus’ (Remote IP: ‘213.55.244.27’)2020-07-07T14:04:18+0200WarningcoreLogin failed: ‘Joel+Niklaus’ (Remote IP: ‘213.55.244.27’)2020-07-07T14:03:55+0200WarningcoreLogin failed: ‘Joel+Niklaus’ (Remote IP: ‘213.55.244.27’)2020-07-07T14:03:53+0200WarningcoreLogin failed: ‘Joel+Niklaus’ (Remote IP: ‘213.55.244.27’)2020-07-07T14:03:30+0200WarningcoreLogin failed: ‘Joel+Niklaus’ (Remote IP: ‘213.55.244.27’)2020-07-07T14:03:05+0200WarningcoreLogin failed: ‘Joel+Niklaus’ (Remote IP: ‘213.55.244.27’)2020-07-07T14:03:05+0200WarningcoreLogin failed: ‘Joel+Niklaus’ (Remote IP: ‘213.55.244.27’)2020-07-07T14:02:40+0200WarningcoreLogin failed: ‘Joel+Niklaus’ (Remote IP: ‘213.55.244.27’)2020-07-07T14:02:40+0200WarningcoreLogin failed: ‘Joel+Niklaus’ (Remote IP: ‘213.55.244.27’)2020-07-07T14:02:40+0200WarningcoreLogin failed: ‘Joel+Niklaus’ (Remote IP: ‘213.55.244.27’)2020-07-07T13:50:58+0200WarningcoreLogin failed: ‘Joel+Niklaus’ (Remote IP: ‘213.55.244.27’)2020-07-07T13:50:33+0200WarningcoreLogin failed: ‘Joel+Niklaus’ (Remote IP: ‘213.55.244.27’)2020-07-07T13:50:33+0200WarningcoreLogin failed: ‘Joel+Niklaus’ (Remote IP: ‘213.55.244.27’)2020-07-07T13:50:16+0200WarningcoreLogin failed: ‘Joel+Niklaus’ (Remote IP: ‘213.55.244.27’)2020-07-07T13:50:08+0200WarningcoreLogin failed: ‘Joel+Niklaus’ (Remote IP: ‘213.55.244.27’)2020-07-07T13:41:37+0200WarningcoreLogin failed: ‘Joel+Niklaus’ (Remote IP: ‘213.55.244.27’)2020-07-07T13:41:12+0200WarningcoreLogin failed: ‘Joel+Niklaus’ (Remote IP: ‘213.55.244.27’)
{“reqId”:“gMMH4YfWGxlYnuFcvyr8”,“level”:2,“time”:“2020-07-07T12:03:53+00:00”,“remoteAddr”:“213.55.244.27”,“user”:"–",“app”:“core”,“method”:“GET”,“url”:"/ocs/v2.php/cloud/user?format=json",“message”:“Login failed: ‘Joel+Niklaus’ (Remote IP: ‘213.55.244.27’)”,“userAgent”:“Mozilla/5.0 (iOS) Nextcloud-iOS/3.0.0”,“version”:“19.0.0.12”}
{“reqId”:“gORWAICbjVvH0qGN6lS0”,“level”:2,“time”:“2020-07-07T12:03:55+00:00”,“remoteAddr”:“213.55.244.27”,“user”:"–",“app”:“core”,“method”:“GET”,“url”:"/index.php/avatar/KD3i2PMZw8CRB87W/128",“message”:“Login failed: ‘Joel+Niklaus’ (Remote IP: ‘213.55.244.27’)”,“userAgent”:“Mozilla/5.0 (iOS) Nextcloud-iOS/3.0.0”,“version”:“19.0.0.12”}
{“reqId”:“3RJilmJ2UjGeCYdckEzV”,“level”:2,“time”:“2020-07-07T12:04:18+00:00”,“remoteAddr”:“213.55.244.27”,“user”:"–",“app”:“core”,“method”:“PROPFIND”,“url”:"/remote.php/webdav",“message”:“Login failed: ‘Joel+Niklaus’ (Remote IP: ‘213.55.244.27’)”,“userAgent”:“Mozilla/5.0 (iOS) Nextcloud-iOS/3.0.0”,“version”:“19.0.0.12”}
{“reqId”:“HPUtbOdBqwpuy8Ou81xu”,“level”:2,“time”:“2020-07-07T12:04:20+00:00”,“remoteAddr”:“213.55.244.27”,“user”:"–",“app”:“core”,“method”:“GET”,“url”:"/index.php/avatar/KD3i2PMZw8CRB87W/128",“message”:“Login failed: ‘Joel+Niklaus’ (Remote IP: ‘213.55.244.27’)”,“userAgent”:“Mozilla/5.0 (iOS) Nextcloud-iOS/3.0.0”,“version”:“19.0.0.12”}
{“reqId”:“FYjvBUFpv4dSRbk9RWFE”,“level”:2,“time”:“2020-07-07T12:04:27+00:00”,“remoteAddr”:“213.55.244.27”,“user”:"–",“app”:“core”,“method”:“SEARCH”,“url”:"/remote.php/dav",“message”:“Login failed: ‘Joel+Niklaus’ (Remote IP: ‘213.55.244.27’)”,“userAgent”:“Mozilla/5.0 (iOS) Nextcloud-iOS/3.0.0”,“version”:“19.0.0.12”}
Joel+Niklaus
Can you test the char between the words?
What should I test exactly?
I am logging in with the email address joel.niklaus@inf.unibe.ch not with the username
Can you add a user with only chars or login with username? Could be problem with username and remote.php
I just tried to login with the username “Joel Niklaus” and “JoelNiklaus”, which both did not work (same problem as before)