Thank you Martin,

while MaxSynRetransmissions=3 did not change the 21s timeout (if I get it right that makes sense) but increasing this to 4 or 5 did work great and the connection did not timeout after 21s as before.

It seems that the 21 seconds is Windows OS limit, that an application cannot override.
See for example:
Where does the socket timeout of 21000 ms come from?

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Anyway, check this post:
Connection/Response timeout values don't seem to take effect in JMeter
It suggests, that you might increase the OS limit using:

Hi Martin,

I have been able to run new tests today and generated an english log file. I hope this helps in determining the cause. Thank you!

Thanks. I could translate the messages myself :) But I need exact original English error messages to investigate this.

Hi Martin,

we have no ability to connect to the satellite network currently so I have attached the translation of the german parts.

We do already run a batch skript that retries the connection, it usually results in the behavior depicted. If we connect successfully, then always slightly below ~15s time but that is hard to achieve with 2400 baud connection.

Please post English log file.
Also, you might consider using this:
Retrying Script/Connection Attempt

Hi Martin,

thanks for the reply, I have attached an examplary log from the last time.

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

Hi there,

we've been trying to run a WinSCP batch script that connects to an FTP server. We transmit via a slow satellite connection (<2400 bit/s) so the biggest problem is getting the connection to the FTP server up, the actual file transfer while connected is ok. In most cases the connection attempt fails after around 15–20 seconds despite setting the connection timeout to 60 seconds. (Still connected to the internet) What might be the reason for this? Is this a problem of our client or the remote server?

Thank you very much.