virtualhost: timeout when the load of a hosted website is balanced as a virtual host

So I have a Haproxy on a Linux server.
Load balanced to three other servers, two are Windows servers and one is a Linux server with Plesk that manages its stuff.

The main difference between the content of the three servers is that Windows servers have the content of the website hosted directly on a port, while on the Linux server, because it has other websites hosted on it and because Plesk admin, I have this website The "version" hosted as a virtual host, as the way I would usually add a domain and such in Plesk.

So, the problem is that there are 404 wait times and errors for the CSS files when it is the turn of the Linux server in the round-robin, while the other servers are doing their job very well. I can not identify the problem, I tried to set the header "Host" as my domain to make sure there is no problem related to the fact that the host header is not sent (so the virtual host mechanism will not work).

Any idea what the problem might be? I really can not host the domain in a different port, because I'm afraid I could face many complexities due to the Plesk configurations.