A website was not working. In the pi-hole GUI, in the query log, the line showing the blocked domain was shown. Using the 'Whitelist' action button on the same line resulted in a pop-up window showing the error message "While executing: attempt to write a readonly database Added 0 out of 1 domains". Using the pi-hole command as super-user did whitelist the aforementioned domain. Trying to blacklist a random domain from the query log did not work when using the GUI, with the same error message.
My expectation is that whitelisting and blacklisting should work both from the command line and from the GUI.
Pi-hole is running on a Raspberry Pi 3. Pi-hole is also providing the DHCP service. The installation is special in one respect: in stead of lighttpd web server nginx is used. My debug token is https://tricorder.pi-hole.net/y2x2reymqj.