I tried the user 'admin' in the WEBUI. The tacacs service is triggered now. I tried another user but as mentioned yet it does not trigger the service at all. As the user 'admin' user is not configured on our tacacs-servers I was not logged in but I could trace the activity:
tcpdump -i any port 49
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on any, link-type LINUX_SLL (Linux cooked v1), capture size 262144 bytes
14:05:48.230704 IP 192.168.0.23.42106 > 10.15.13.56.49: Flags [S], seq 2906401710, win 64620, options [mss 1436,sackOK,TS val 1884355110 ecr 0,nop,wscale 5], length 0
14:05:48.274391 IP 10.15.13.56.49 > 192.168.0.23.42106: Flags [S.], seq 151795552, ack 2906401711, win 28960, options [mss 1460,sackOK,TS val 4056467022 ecr 1884355110,nop,wscale 7], length 0
14:05:48.274835 IP 192.168.0.23.42106 > 10.15.13.56.49: Flags [.], ack 1, win 2020, options [nop,nop,TS val 1884355155 ecr 4056467022], length 0
14:05:48.275045 IP 192.168.0.23.42106 > 10.15.13.56.49: Flags [P.], seq 1:13, ack 1, win 2020, options [nop,nop,TS val 1884355155 ecr 4056467022], length 12
14:05:48.317364 IP 10.15.13.56.49 > 192.168.0.23.42106: Flags [.], ack 13, win 227, options [nop,nop,TS val 4056467065 ecr 1884355155], length 0
14:05:48.317620 IP 192.168.0.23.42106 > 10.15.13.56.49: Flags [P.], seq 13:46, ack 1, win 2020, options [nop,nop,TS val 1884355197 ecr 4056467065], length 33
14:05:48.360379 IP 10.15.13.56.49 > 192.168.0.23.42106: Flags [.], ack 46, win 227, options [nop,nop,TS val 4056467108 ecr 1884355197], length 0
14:05:48.408403 IP 10.15.13.56.49 > 192.168.0.23.42106: Flags [P.], seq 1:33, ack 46, win 227, options [nop,nop,TS val 4056467156 ecr 1884355197], length 32
14:05:48.408412 IP 10.15.13.56.49 > 192.168.0.23.42106: Flags [F.], seq 33, ack 46, win 227, options [nop,nop,TS val 4056467156 ecr 1884355197], length 0
14:05:48.408652 IP 192.168.0.23.42106 > 10.15.13.56.49: Flags [.], ack 33, win 2019, options [nop,nop,TS val 1884355289 ecr 4056467156], length 0
14:05:48.409021 IP 192.168.0.23.42106 > 10.15.13.56.49: Flags [F.], seq 46, ack 34, win 2019, options [nop,nop,TS val 1884355289 ecr 4056467156], length 0
14:05:48.455382 IP 10.15.13.56.49 > 192.168.0.23.42106: Flags [.], ack 47, win 227, options [nop,nop,TS val 4056467203 ecr 1884355289], length 0
14:06:00.755044 IP 192.168.0.23.60504 > 10.15.13.56.49: Flags [S], seq 3406943260, win 64620, options [mss 1436,sackOK,TS val 1884367635 ecr 0,nop,wscale 5], length 0
14:06:00.791433 IP 10.15.13.56.49 > 192.168.0.23.60504: Flags [S.], seq 1106505115, ack 3406943261, win 28960, options [mss 1460,sackOK,TS val 4056479539 ecr 1884367635,nop,wscale 7], length 0
14:06:00.791795 IP 192.168.0.23.60504 > 10.15.13.56.49: Flags [.], ack 1, win 2020, options [nop,nop,TS val 1884367672 ecr 4056479539], length 0
14:06:00.792365 IP 192.168.0.23.60504 > 10.15.13.56.49: Flags [P.], seq 1:13, ack 1, win 2020, options [nop,nop,TS val 1884367672 ecr 4056479539], length 12
14:06:00.835483 IP 10.15.13.56.49 > 192.168.0.23.60504: Flags [.], ack 13, win 227, options [nop,nop,TS val 4056479583 ecr 1884367672], length 0
14:06:00.835723 IP 192.168.0.23.60504 > 10.15.13.56.49: Flags [P.], seq 13:46, ack 1, win 2020, options [nop,nop,TS val 1884367716 ecr 4056479583], length 33
14:06:00.878489 IP 10.15.13.56.49 > 192.168.0.23.60504: Flags [.], ack 46, win 227, options [nop,nop,TS val 4056479626 ecr 1884367716], length 0
14:06:00.885522 IP 10.15.13.56.49 > 192.168.0.23.60504: Flags [P.], seq 1:33, ack 46, win 227, options [nop,nop,TS val 4056479633 ecr 1884367716], length 32
14:06:00.885721 IP 192.168.0.23.60504 > 10.15.13.56.49: Flags [.], ack 33, win 2019, options [nop,nop,TS val 1884367766 ecr 4056479633], length 0
14:06:00.886346 IP 192.168.0.23.60504 > 10.15.13.56.49: Flags [F.], seq 46, ack 33, win 2019, options [nop,nop,TS val 1884367766 ecr 4056479633], length 0
14:06:00.886366 IP 10.15.13.56.49 > 192.168.0.23.60504: Flags [F.], seq 33, ack 46, win 227, options [nop,nop,TS val 4056479633 ecr 1884367716], length 0
14:06:00.886700 IP 192.168.0.23.60504 > 10.15.13.56.49: Flags [.], ack 34, win 2019, options [nop,nop,TS val 1884367766 ecr 4056479633], length 0
14:06:00.936530 IP 10.15.13.56.49 > 192.168.0.23.60504: Flags [.], ack 47, win 227, options [nop,nop,TS val 4056479684 ecr 1884367766], length 0