You upgraded to Nextcloud 25, right? This is a known issue with 32bit OS:
https://github.com/nextcloud/server/issues/34674
There is a fix available: https://help.nextcloud.com/t/cannot-access-files-after-upgrade-to-nextcloud-25/147823/6?u=michaing
Or you change to the ARMv8 (64bit) image, but this needs more effort and time, but it is more future proof.