After a serious upgrade of my NextCloud server, I had the strange phenomenon that my files view was completely empty. Tried everything, nothing worked. In the end the cause was proven to be an extra folder in de data directory of Nextcloud.
I had copied all data from a usb drive and put it in a __filesfrompreviousserver folder in the /var/snap/nextcloud/common/nextcloud/data folder, which is the folder where NextCloud stores its data.
I never thought that would cause problems, but apparently it does. Once I removed the folder everything was fine again.