In /var/www/vhosts/system/mydomain.com/conf/httpd.conf I see this Alias:
Alias "/plesk-stat" "/var/www/vhosts/system/mydomain.com/statistics"
<Location /plesk-stat/>
Options +Indexes
</Location>
<Location /plesk-stat/logs/>
Require valid-user
</Location>
Alias /webstat /var/www/vhosts/system/mydomain.com/statistics/webstat
Alias /webstat-ssl /var/www/vhosts/system/mydomain.com/statistics/webstat-ssl
Alias /ftpstat /var/www/vhosts/system/mydomain.com/statistics/ftpstat
Alias /anon_ftpstat /var/www/vhosts/system/mydomain.com/statistics/anon_ftpstat
Alias /awstats-icon /usr/share/awstats/icon
I suspect that the Aliases and the RewriteRule don't go well together:
These too throw a 404.
Sub-domains (without Plone content) do show statistics with plesk-stat/webstat. These URLs are generated by Plesk and that works as long as you don't rewrite the URL as is needed with a Plone site.
Well, I couldn't solve this issue but I've found a workaround.
I created a sub-domain and in the "Apache & nginx Settings" (in Plesk) of that sub-domain I added "Alias /webstat-ssl /var/www/vhosts/system/mydomain.com/statistics/webstat-ssl".
So, now I can watch the statistics from my Plone site on that sub-domain.