The solution has been found in the german post about the same bug.
(Nextcloud Android App Version 3.32 - Automatischer Fotoupload "Lokale Datei nicht gefunden" / Auto Upload: Local File not found - #4 by maziarz189)
You have to give Nextcloud access to “All file access”
The easiest way is to go to your Settings and search “All file access” and look for Nextcloud and check the toggle.
Otherwise you can Navigate via: Settings->Apps->(3 dot menu on top)->Special Access->All files access