You have enabled Use scp2 with scp1 compat. session option. Disable it.
https://winscp.net/eng/docs/ui_login_scp

Dear Matin

Thank you for your attention and support. Attached, a log file (debug level 2) showing results for a two connections to ssh (openssh-7.6 servers): the first one (to matata, a 11.1-FreeBSD server) is that one with problems to SCP transfer and, the second one, to celhart (10.4-FreBSD server, openssh-7.6 server) is that one wheres no problems happened.

My best wishes

Eduardo

Please attach a full session log file showing the problem (using the latest version of WinSCP).

Dear

I have no problems to transfer to connect my laptop (win10, latest WinSCP release) to a FreeBSD server (11.1-RELEASE) where openssh-7.6 is the daemon active using SCP protocol .
But, when I tried to transfer files, I got:

Cannot execute SCP to start transfer. Please make sure that SCP is installed on the server and path to it is included in PATH. You may also try SFTP instead of SCP.
Command failed with return code 255.

I checked, scp is on $PATH.

However, when I connect to another FreBSD server (10.4-RELEASE), using SCP protocol, everything is ok. Further, when I try to transfer from a FreeBSD (10.4) server to FreeBSD (11.1) server, everything goes right. Thus, I concluded that there is no problem in FreeBSD (11.1) to receive files using SCP protocol.

When I changed protocol in WinSCP (from SCP to SFTP) in my laptop, I can transfer to FreeBSD(11.1) server without problems. My conclusion is there is a problem with SCP protocol in WinSCP with openssh-7.6 under FreeBSD(11.1) server.

Please, could you give me a hint to solve this problem?

Thanks in advance for your attention

Yours

Eduardo

Here, server information:

File transfer protocol = SFTP-3
Cryptographic protocol = SSH-2
SSH implementation = OpenSSH_7.6
Encryption algorithm = aes
Compression = No
------------------------------------------------------------
Server host key fingerprint
ssh-rsa 2048 81:35:14:0b:2d:20:79:45:ee:ef:13:46:43:49:5f:6d
------------------------------------------------------------
Can change permissions = Yes
Can change owner/group = Yes
Can execute arbitrary command = No
Can create symbolic/hard link = Yes/Yes
Can lookup user groups = No
Can duplicate remote files = No
Can check available space = Yes
Can calculate file checksum = No
Native text (ASCII) mode transfers = No
------------------------------------------------------------
Additional information
The server supports these SFTP extensions:
posix-rename@openssh.com="1"
statvfs@openssh.com="2"
fstatvfs@openssh.com="2"
hardlink@openssh.com="1"
fsync@openssh.com="1"
------------------------------------------------------------
Total bytes on device = 215,482 GB (231,372,612,435,968 B)
Free bytes on device = 208,357 GB (223,722,546,921,472 B)
Total bytes for user = Unknown
Free bytes for user = 208,357 GB (223,722,546,921,472 B)
Bytes per allocation unit = 128 KB (131,072 B)