That cannot be last 20 lines. There must be more. Make sure you dismiss any error message (or even close WinSCP), before collecting the log file.

What do you mean by "Tectia server can connect from OpenSSH scp protocol"? Do you mean "Tectia client can connect with SCP protocol"? What client? A command-line client? Note that a command-line SCP client has far lower requirements on a server than a GUI SCP client like WinSCP.
See https://winscp.net/eng/docs/requirements#scp

Is the setting on the WinSCP side insufficient?
Below is the last 20 lines of the output log.

---------------------------------
. 2017-09-04 19: 31: 39.980 Host key matches cached key
. 2017-09-04 19: 31: 39.980 Initialised AES - 256 SDCTR client -> server encryption
. 2017-09-04 19: 31: 39.980 Initialized HMAC - SHA - 256 client -> server MAC algorithm
. 2017-09-04 19: 31: 39.980 Initialised AES - 256 SDCTR server -> client encryption
. 2017-09-04 19: 31: 39.980 Initialized HMAC - SHA - 256 server -> client MAC algorithm
. 2017-09-04 19: 31: 39.994 Reading key file "C: \ Program Files (x86) \ WinSCP \ key \ Private_key.ppk"
! 2017-09-04 19: 31: 39.994 Using username "<Username>".
. 2017-09-04 19: 31: 41.542 Server offered these authentication methods: password, publickey, keyboard-interactive
. 2017-09-04 19: 31: 41.542 Offered public key
. 2017-09-04 19: 31: 41.542 Offer of public key accepted
! 2017-09-04 19: 31: 41.542 Authenticating with public key "rsa-key"
. 2017-09-04 19: 31: 41.680 Sent public key signature
. 2017-09-04 19: 31: 41.694 Access granted
. 2017-09-04 19: 31: 41.694 Opening session as main channel
. 2017-09-04 19: 31: 41.694 Opened main channel
. 2017-09-04 19: 31: 41.790 Started a shell / command
. 2017-09-04 19: 31: 41.806 --------------------------------------- -----------------------------------
. 2017-09-04 19: 31: 41.806 Using SCP protocol.
. 2017-09-04 19: 31: 41.806 Doing startup conversation with host.
. 2017-09-04 19: 31: 41.870 Skipping host startup message (if any).
> 2017-09-04 19: 31: 41.870 echo "WinSCP: this is end-of-file: 0"
<2017-09-04 19: 31: 41.870 Last login: Mon Sep 04 2017 17: 43: 06 + 0900
---------------------------------

configuration:
SSH Server: Tectia Server 6.4.15 (Windows Server 2016)
SSH Client: WinSCP 5.9.6 (Windows Server 2016)

etc:
1) Logging in with the public key method. Passphrase is unused.
2) Ather information. Tectia server can connect from OpenSSH scp protocol.