An initial test looks promising. But I have to wait and see what happens when the real transfer takes place.
I don't bother right now with the locked file situation. It may become obsolete anyway if the timeout fix works. Otherwise I would come back to it.

The default timeout is 15 seconds:

. 2020-10-16 19:35:51.679 Ping type: Dummy, Ping interval: 30 sec; Timeout: 15 sec

Try 60 seconds.
I do not know about the retry. Did you check what process has the file locked?

Thanks Martin for your quick reply, I wasn't aware that the command-line /timeout has no effect. I will test it with the open switch -timeout=<sec>. What is the default timeout without switch, what is a reasonable value to set?

What about the retry problem "The process cannot access the file because it is being used by another process."
Is there something that can be improved? To retry could helpful in some cases.

Did you try setting longer session timeout? Use -timeout switch of the open command:
https://winscp.net/eng/docs/scriptcommand_open#timeout
(command-line /timeout switch has no effect here)

If that does not help, we need a session log file on the "Normal" level.

We have five monitoring stations which send three times a day a 200k zip file to a ftp server using a batch file and WinSCP. This works perfectly fine. However, every 9 days the zip file is larger (30 MB) and one station runs into trouble. This station connects to the internet over the mobile network. This connection isn't perfectly good but not bad either. At least we manage to access and control the station with TeamViewer.

To send the zip file the scheduler calls a batch file which contains this line:
CALL "%ProgramFiles%\WinSCP\winscp.com" /timeout=120 /ini=nul /script=ftp.txt /log="ftp.log" /loglevel=-1 /logsize=1M

The script file ftp.txt contains the following lines:

The problem starts with this:

Script: Timeout detected. (control connection)

It seems to re-try but fails with this error:

The process cannot access the file because it is being used by another process.

I have attached the log file. Is there something I can do better, to make a successful transfer of a larger file with this station?

Any help is appreciated.