if you guys would use the search-option here in the forum (e.g. with “rainbow”) there are some results…
the most promising one is (a bit older) Nextcloud box does not boot on rasberry pi
what i understood from it your problems could come from a broken sd-card. so if you have the opportunity to try it with a different sd?
and the original image? you will find it here --> https://download.nextcloud.com/server/images/
since the both of you got yourself a nextcloud box i think you could rely upon your power-supply which otherwise could be a reason for problems as well.
awww. and if you wouldn’t like the NC snap i suggest: try nextcloudpi. very easy set up as well and highly maintained