Hello there,
i’m not that Linux Guru but in an extreme effort in 2018 i managed it to get NCP running on a Pi3. 2021 I failed to upgrade to Buster and the php got uninstalled. During the holidays a had time to work on the broken NCP installation, so i reinstalled php and made a backup of the NCP using the TUI. I took the current image of NCP and installed it on a new sd-card, restored the backup by using the TUI, but had to copy some files from the user directories to the new instance by using mc. Now the calendar is working and the contacts seem to work. If I connect my smartphone (Samsung S8+) with the NextCloud app (and the davx5 app) from f-droid to my NCP, I see all the files I have copied by mc so far so good. But if I upload a file to the NCP with my smartphone the filename and its extension are lost, in the NCP I see a file with a number with no extension. When I login to my NCP with the Firefox I can upload files without problems, but I don’t see the files I copied with mc nor the files i uploaded with my smartphone. Otherwise on my smartphone i don’t see the files I uploaded with the Firefox. 
Any ideas what to do?
I just noticed that if I create a new note with the android app Notizen for NextCloud i receive an error “TokenMismatchException” The aothetication token mismatches or the package mane has no permission for this task. Seems that there are more problems that I tought or it’s the reason for all the other problems I decribed already. Do s.o. has an idea how to fix it?
Maybe you can limit the error to the smartphone or the cloud.
a.) Can you use another android device. Does it work with your nextcloud?
b.) Can you use another nextcloud?
You can get a 60 minutes test account at https://try.nextcloud.com .
The user-id is auto-generated 16 charaters long.
The password is demo
The server is https://demo1.nextcloud.com or https://demo2.nextcloud.com
Version is Nextcloud 21.0.4 .