Hi rafm,

Page 124 https://dell.to/3i7MW83 covers how to create a VLAN to separate the traffic from other devices. Let us know if you have any additional questions.

I had already seen this pdf.
I tried the following setup without success.

What would be the correct form?

Untagged is most likely what you want to use. https://dell.to/33ssHhs

I tried with the T and U unsuccessfully.
I am still unable to perform this configuration.
Regardless of the configuration (T / U) everyone continues to access everyone.

Are the clients are in a different VLAN?

Yes 

vlan1  Computers can access vlan 40, and vice versa.

I created a vlan 50, but it didn't solve 

Ok, so they all can communicate so the issue is that you want to limit access to the server? Where are they pointing for their default gateway? Something is routing them to vlan 40 from vlan 1.

Yes, just desktops from one sector can access the server.

This server have a static IP, rest of network DHCP from a Linux server.

And desktops from this sector need access rest of the network (internet, printers, other servers...)

I never worked with vlans, this switch just have a 2 ports LAG with a 2824

You need to make it so that the server can’t reach the clients you don’t want it to, it probably is changing the routing on the clients to not let it route to vlan 40 and possibly make it so the clients are not on vlan 1 so that the server can't reach them on the default vlan. 

Sorry but i didn't understand what i should do

The problem is something is routing the VLANs to be able to communicate, it isn’t with this switch.

We use isc dhcpd server configured with one IP range .

The dhcpd is the gateway , DNS is from  our iSP.