Support intro
Sorry to hear you’re facing problems 
help.nextcloud.com is for home/non-enterprise users. If you’re running a business, paid support can be accessed via portal.nextcloud.com where we can ensure your business keeps running smoothly.
In order to help you as quickly as possible, before clicking Create Topic please provide as much of the below as you can. Feel free to use a pastebin service for logs, otherwise either indent short log examples with four spaces:
example
Or for longer, use three backticks above and below the code snippet:
longer
example
here
Some or all of the below information will be requested if it isn’t supplied; for fastest response please provide as much as you can 
Nextcloud version (eg, 20.0.5): 21.0.3
Operating system and version (eg, Ubuntu 20.04): Ubuntu 20.04.2
Apache or nginx version (eg, Apache 2.4.25): 2.4.41-4ubuntu3.3
PHP version (eg, 7.4): 7.5
Android client version: 30160190
I’ve been hit by the bug here: Parameter specified as non-null is null · Issue #8370 · nextcloud/android · GitHub
It happened when I upgraded my server to Nextcloud 20, and persisted after upgrading to 21. Rolling back to pre-20 seems like a really bad thing to try.
Can I find older APKs for the Android client? Some info on github indicates that the 3.15 version still works.