That’s what I resigned myself to doing: reinstalling the application with an empty backup folder.
The error on line 59672 had nothing to do with my problem, as I understood from reading an interesting discussion on the subject.
On the other hand, the presence of json arrays like this one {'value': '/etc;/home;/root'} in place of the valid information /etc; /root; /home contaminated all Urbackup clients outside the local network! 
For example, on a client not in the local network, the authentication key is corrupted:
root@Nextcloud:~ # grep auth /usr/local/var/urbackup/data/settings.cfg
internet_authkey.group={'value': 'Fg5YCmMtyV'}
internet_authkey=FEMk9To90l
but on a local network client, the authentication key is valid:
root@checkmk:~ # grep auth /usr/local/var/urbackup/data/settings.cfg
internet_authkey.group=AVtiGCynzU
internet_authkey=EU4ccjaAeH
I think the urbackup_api_test.py script has definitely corrupted the settings table data in the backup_server_settings.db database… Unfortunately, I didn’t have a backup of the Urbackup application databases.
Ernest.