i believe that this option is only available for compatibility with other mail servers other than Nethservice.
Certainly @gabriele_bulfon will have some more information on how to use this "Alternate root folder" option.
To resolve this issue and return to the default setting, just delete the straight line directly on the db as follows:
su - postgres -c 'psql webtop5'
delete from core.user_settings where key='defaultfolder';
I recommend that you leave from WebTop5 avoiding going back with WebTop4
To reinstall WebTop5 you must remove the WebTop4 packages:
systemctl stop tomcat
yum remove webtop4-libs webtop4-zpush nethserver-tomcat
rm -rf /var/lib/tomcat/webapps/webtop
and clean the db:
su - postgres
Remove additional fragments:
rm -rf /usr/share/webtop /var/lib/nethserver/secrets/webtop
Reinstall WebTop 5:
yum install @nethserver-webtop5-groupware
I hope to be proved helpful...
Now I go back to the beach