Many 404 URLs when using mod_proxy[_html]

What I want to do

Make all of http://cloud.example.com/ appear as http://webmail.example.com/cloud/

• http://cloud.example.com : the targeted website
• http://webmail.example.com : the website where apache2 is running with mod_proxy, as reverse proxy (but not exclusively, there's also webmail).
• http://webmail.example.com/cloud : the URL under which I would like http://cloud.example.com to appear.

How I tried to do it

<VirtualHost  *:80>

  ServerName webmail.example.com
  DocumentRoot /var/www/roundcubemail-1.1.4
  Loglevel debug
  ErrorLog /var/log/apache2/roundcube.error
  # %t  : Time 
  # %a  : client ip 
  # %f  : filename
  # %T  : processing time 
  # %s  : status
  # %>s : Final status
  LogFormat "%t %a %{Referer}i -> %U%q, matches %f, %s>%>s(served in %{ms}T µs)" custom
  # was "combined"
  CustomLog /var/log/apache2/roundcube.access custom     

  ProxyPass  "/cloud/" "http://cloud.example.com/"
  ProxyPassReverse "/cloud/" "http://cloud.example.com/"

  # The ProxyRequests directive should usually be set off when using ProxyPass.
  # src:https://httpd.apache.org/docs/2.4/mod/mod_proxy.html#proxypass
  ProxyRequests off

  # https://httpd.apache.org/docs/current/mod/mod_proxy.html#proxypreservehost  
  # This option should normally be turned Off.
  ProxyPreserveHost off

</VirtualHost>

Logs

[14/Jun/2016:13:42:37 +0100] 192.168.211.202 - -> /cloud/, matches proxy:http://cloud.example.com/, 200>200(served in 5 µs)
[14/Jun/2016:13:42:42 +0100] 192.168.211.202 - -> /core/css/styles.css?v=9a1b1350ff46cbc098bc6c7acca55c1c, matches /var/www/roundcubemail-1.1.4/core, 404>404(served in 0 µs)
[14/Jun/2016:13:42:42 +0100] 192.168.211.202 - -> /core/css/header.css?v=9a1b1350ff46cbc098bc6c7acca55c1c, matches /var/www/roundcubemail-1.1.4/core, 404>404(served in 0 µs)
[14/Jun/2016:13:42:42 +0100] 192.168.211.202 - -> /core/css/mobile.css?v=9a1b1350ff46cbc098bc6c7acca55c1c, matches /var/www/roundcubemail-1.1.4/core, 404>404(served in 0 µs)
[14/Jun/2016:13:42:42 +0100] 192.168.211.202 - -> /core/css/icons.css?v=9a1b1350ff46cbc098bc6c7acca55c1c, matches /var/www/roundcubemail-1.1.4/core, 404>404(served in 0 µs)
[14/Jun/2016:13:42:42 +0100] 192.168.211.202 - -> /core/css/fonts.css?v=9a1b1350ff46cbc098bc6c7acca55c1c, matches /var/www/roundcubemail-1.1.4/core, 404>404(served in 0 µs)
[14/Jun/2016:13:42:42 +0100] 192.168.211.202 - -> /core/css/apps.css?v=9a1b1350ff46cbc098bc6c7acca55c1c, matches /var/www/roundcubemail-1.1.4/core, 404>404(served in 0 µs)
...

First line of the log shows that requesting the "/cloud/" path successfully retrieves the ressource from the the proxied website, but all subsequent ressources aren't fetched from the cloud server but from the webmail server itself.

How can I take care of this ?

I have proxy_html enabled.

root@messagerie[CHROOT][10.10.10.20] ~ # a2query -m
proxy (enabled by site administrator)
php5 (enabled by maintainer script)
setenvif (enabled by maintainer script)
ssl (enabled by site administrator)
deflate (enabled by maintainer script)
authn_core (enabled by maintainer script)
env (enabled by maintainer script)
mime (enabled by maintainer script)
auth_basic (enabled by maintainer script)
access_compat (enabled by maintainer script)
authz_core (enabled by maintainer script)
xml2enc (enabled by site administrator)
authz_host (enabled by maintainer script)
alias (enabled by maintainer script)
authz_user (enabled by maintainer script)
autoindex (enabled by maintainer script)
negotiation (enabled by maintainer script)
socache_shmcb (enabled by site administrator)
proxy_http (enabled by site administrator)
status (enabled by maintainer script)
proxy_html (enabled by site administrator)
filter (enabled by maintainer script)
authn_file (enabled by maintainer script)
dir (enabled by maintainer script)
mpm_prefork (enabled by maintainer script)
root@messagerie[CHROOT][10.10.10.20] ~ #