After switching webmail client from Horde to Roundcube, webmail shows the error below:
The requested URL /roundcube/index.php was not found on this server.
Apache Server at webmail.domain.tld Port 80
Horde configuration file is not deleted after switching to Roundcube. This is Plesk internal issue #PPPM-1339 which will be fixed in future product updates.
/etc/httpd/conf/plesk.conf.d/webmails/horde/domainXXX_webmail.conf file manually.