Why can't a TCP connection be established?

Question

linux

Actually there are 2 virtual machines with ubuntu. If "a separate internal network" for their connection. Suddenly, the connection began to work stably with failures (either pings do not pass, then the TCP connection is not established).
Moreover, I took the logs through tcpdump and what I see there .... "Machine 2" tries to establish a connection, it is refused ... and in the logs from the first machine there is no connection attempt at all.
And "before everything worked fine".
upd.
Virtual machines are located on one physical machine.
Their connection is made through an "isolated network" (in virt-manager's terminology) with rtl8139.
upd. 2

17 7.700152 192.168.254.2 192.168.254.1 TCP 74 48902> 48654 [SYN] SEQ = 0 Win = 42340 Len = 0 MSS = 1460 SACK_PERM = 1 TSVAL = 949350 TSECR = 0 WS = 4096
18 7.700399 192.168.254.1 192.168.254.2 TCP 60 48654 > 48902 [RST, ACK] Seq=1 Ack=1 Win=0 Len=0
19 7.700503 192.168.254.2 192.168.254.1 TCP 74 41203 > 48654 [SYN] Seq=0 Win=42340 Len=0 MSS=1460 SACK_PERM= 1 TSval=949350 TSecr=0 WS=4096
20 7.700635 192.168.254.1 192.168.254.2 TCP 60 48654 > 41203 [RST, ACK] Seq=1 Ack=1 Win=0 Len=0
//Same thing.

Answer 1

1.dhcp?
2.vlan?
3.firewall?
4. network config on the host node?
5. loading of the physical interface?

Answer 2

libvirt has a tendency to install a firewall. I usually start the necessary bridge myself and connect virtual machines to it. Well, I also recommend using virtio if the guest system supports it (and they all support it now, even there are drivers for Windows).