Why don't computers get an IP address?

Question

System administration

Initially, IP addresses were distributed by a DHCP server implemented in the Usergate proxy server. All of a sudden it stopped issuing IP addresses to clients. I disabled this DHCP server and on the same server raised a Windows Server DHCP server. Wireshark shows that the DHCP Offer message does not reach the client, whichever of the two DHCP servers is active. Firewall is disabled. Kaspersky is unloaded.
The network is flat. For the test, I took out a new unmanaged switch, plugged only the server and client into it, and nothing has changed. In the DHCP server snap-in, I see that 5 out of 50 clients still received IP addresses.
Dumps from the server and client are here: https://yadi.sk/d/1IrgR0MFIsMt0Q https://yadi.sk/d/aZVZLPDoetRGoA(applied capture filter "port 67 || port 68"). I mirrored the switch port in which the server is included, this is a dump from it https://yadi.sk/d/KU9MExfoxzgE6A (the capture filter "eth.port == C8-D3-A3-AD-20-58" is applied - this is the client) . It can be seen that the DHCP Offer did not reach the switch.
This Usergate has a firewall. But there are rules that allow incoming and outgoing connections on the local network for any traffic. Nobody changed them, and everything worked for months, until, suddenly, clients stopped receiving IP addresses.
What could be the problem? I checked everything I could, I have no more ideas.
Update: I noticed that if you stop the Usergate service, then the addresses are normally obtained.
Those. the culprit is Usergate. I haven't figured out the reason yet. No one allegedly made any changes to its configuration.
Update 2: Added rules for ports 67 and 68 to the firewall, and it worked. I have no explanation why it suddenly stopped working. Maybe they are deceiving me, saying that no one touched anything.

Answer 1

Kaspersky disable and check.

Answer 2

If everything worked and suddenly stopped, why can't you take win serv , a stupid switch and connect 1-2 PCs directly. If dhcp does not work, maybe it's all about the network card? Or is there somewhere else in the switch office behind the wall, which is faulty, so 5 PCs work, but the rest do not.