Multiple VLANs/Subnets

we have several VLANs/Subnets and the backup server has a network connection to every subnet, so that the client could see the server.

I have some clients in the 192.168.15.x sub net and they see the broadcast from the server 192.168.15.72:
1 0.000000 192.168.15.72 255.255.255.255 UDP 60 35623 → 35622 Len=1

and the server is directly connecting to the client
2 0.000024 192.168.15.72 192.168.15.152 UDP 60 35623 → 35622 Len=1

But I also see, that the server is trying to reach this client from his other network adapters:
3 0.000057 192.168.70.72 192.168.15.152 UDP 60 35623 → 35622 Len=1
4 0.000144 192.168.50.72 192.168.15.152 UDP 60 35623 → 35622 Len=1
5 0.000145 192.168.40.72 192.168.15.152 UDP 60 35623 → 35622 Len=1
6 0.000146 192.168.30.72 192.168.15.152 UDP 60 35623 → 35622 Len=1

so the client tries to respond to the server on the wrong subnet (70):
7 0.236490 192.168.15.152 192.168.70.72 UDP 50 35622 → 35623 Len=8

Why does the server try to reach the client over ALL network cards?

He has a correct routing table:
Ziel Router Genmask Flags MSS Fenster irtt Iface
default router4.intern. 0.0.0.0 UG 0 0 0 ens160
192.168.15.0 0.0.0.0 255.255.255.0 U 0 0 0 ens160
192.168.30.0 0.0.0.0 255.255.255.0 U 0 0 0 ens192
192.168.40.0 0.0.0.0 255.255.255.0 U 0 0 0 ens224
192.168.50.0 0.0.0.0 255.255.255.0 U 0 0 0 ens161
192.168.70.0 0.0.0.0 255.255.255.0 U 0 0 0 ens256

and knows that he could reach clients in the .15 subnet over interface ens160

I checked it on the server side: The server is sending this UDP packets over the wrong interface !

root@UrBackup-VM02:/tmp# tcpdump -i ens160 -n -v host 192.168.15.152
tcpdump: listening on ens160, link-type EN10MB (Ethernet), capture size 262144 bytes
17:40:25.635741 IP (tos 0x0, ttl 128, id 299, offset 0, flags [none], proto UDP (17), length 68)
192.168.15.152.57015 > 255.255.255.255.1947: UDP, length 40
17:40:29.681717 IP (tos 0x0, ttl 128, id 8143, offset 0, flags [none], proto UDP (17), length 68)
192.168.15.152.57015 > 192.168.15.255.1947: UDP, length 40
17:40:30.688201 IP (tos 0x0, ttl 64, id 43848, offset 0, flags [DF], proto UDP (17), length 29)
192.168.15.72.35623 > 192.168.15.152.35622: UDP, length 1
17:40:30.688240 IP (tos 0x0, ttl 64, id 43224, offset 0, flags [DF], proto UDP (17), length 29)
192.168.70.72.35623 > 192.168.15.152.35622: UDP, length 1
17:40:30.688272 IP (tos 0x0, ttl 64, id 21741, offset 0, flags [DF], proto UDP (17), length 29)
192.168.50.72.35623 > 192.168.15.152.35622: UDP, length 1
17:40:30.688305 IP (tos 0x0, ttl 64, id 44449, offset 0, flags [DF], proto UDP (17), length 29)
192.168.40.72.35623 > 192.168.15.152.35622: UDP, length 1
17:40:30.688338 IP (tos 0x0, ttl 64, id 28629, offset 0, flags [DF], proto UDP (17), length 29)
192.168.30.72.35623 > 192.168.15.152.35622: UDP, length 1

root@UrBackup-VM02:/tmp# ifconfig ens160
ens160: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 192.168.15.72 netmask 255.255.255.0 broadcast 192.168.15.255
ether 00:0c:29:1d:cf:2f txqueuelen 1000 (Ethernet)
RX packets 1079266346 bytes 3403472946350 (3.4 TB)
RX errors 0 dropped 2671 overruns 0 frame 0
TX packets 612743782 bytes 2772018295067 (2.7 TB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0

root@UrBackup-VM02:/tmp# netstat -r
Kernel-IP-Routentabelle
Ziel Router Genmask Flags MSS Fenster irtt Iface
default router4.intern. 0.0.0.0 UG 0 0 0 ens160
192.168.15.0 0.0.0.0 255.255.255.0 U 0 0 0 ens160
192.168.30.0 0.0.0.0 255.255.255.0 U 0 0 0 ens256
192.168.40.0 0.0.0.0 255.255.255.0 U 0 0 0 ens224
192.168.50.0 0.0.0.0 255.255.255.0 U 0 0 0 ens193
192.168.70.0 0.0.0.0 255.255.255.0 U 0 0 0 ens161
root@UrBackup-VM02:/tmp#

::::::::::::::
/etc/lsb-release
::::::::::::::
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=18.04
DISTRIB_CODENAME=bionic
DISTRIB_DESCRIPTION=“Ubuntu 18.04.3 LTS”

root@UrBackup-VM02:/tmp# urbackupsrv --version
UrBackup Server v2.4.11.0