Pentaho data integration, Move Files ERROR: Could not rename file

Question

I'm running a job with Pentaho Data Integration 8.1, I retrieve a file frop ftp, work on it and move it to a folder. I tried, as I did many other times with no problems, to use 'Move Files', but I get this error:

 Move Files - ERROR (version 8.1.0.0-365, build 8.1.0.0-365 from 2018-04-30 09.42.24 by buildguy) : There was an error moving file [file:///C:.../file.csv] to [file:///C:.../file.csv] : [Could not rename "file:///C:.../file.csv" to "file:///C:.../file.csv".

I checked permissions on the file and on the destination folder, everything's fine.
I tried to use "Process result filenames" step with no success.
I tried to rename or delete the file instead of moving it, same error.
I tried to move this job into a new parent one, with "Move Files" step in it, same error again.
I also tried not including the filename to result prior to the Move Files step: nothing.

Then I wrote a simple shell script:

cd\
cd C:\...\working_directory
move "filename with spaces.csv" /destination_folder

and I get this (I translated it from italian):

ERROR (version 8.1.0.0-365, build 8.1.0.0-365 from 2018-04-30 09.42.24 by buildguy) : (stderr) No access to the file. The file Š is used in another process.

The file isn't used in any other process, I can move it manually without the spoon process running, so I think the ftp step, or the transformation one I use to work on the file (simple 'CSV Input' and ETL) somehow keep the file open. Any idea as to how to "unlock" the file, so that it can be moved?