The CardBook log is not written on disk, it is only in memory. The Cardbook log is more verbose in debug mode. This mode may be set in CardBook preferences, and if you choose this mode, think also to increase the log size (usually 50000) also in CardBook preferences. The debug mode is required if you want to debug the server responses from a failed validation.
it is possible to copy all or some lines from the log with the button
Copy to Clipboard.