we do not provide a nextcloud addon, from where did you installed it ?
I installed it from the linuxservices.io repository, following these instructions: RE: Docker + Nextcloud + External storage
But now I've repeated the installation and the problem went away. I suspect that the error occurred because I rebooted my RPI3 too early: once the AddOn gets installed, docker pulls the nextcloud image in the background, and this takes some time (and is not something you can see in the UI). If you reboot your device before this is completed, then the installation will be missing some files.