The database was actually created, but could not be added to KeePassium automatically. (The reason of this still remains a mystery.) As a workaround, add the created database as an existing file.
If KeePassium cannot find the database after the file was saved by KeePassXC, open KeePassXC preferences → General → Basic Settings → File Management → find the option “Safely save database files (maybe incompatible with Dropbox, etc)” → turn it off.
Last Updated: 2020-08-12