oh, somehow I missed that, sorry. And thanks for your great support here in the forum!

The "keepuptodate" command has -filemask switch:
https://winscp.net/eng/docs/scriptcommand_keepuptodate

Ah, didn't know that, thanks! However I meanwhile change to winscp scripts and on https://winscp.net/eng/docs/scripting#commands I couldn't find any file filtering option - did I miss something or is there no command line version of this setting? Thank you!

You can use file mask to limit syncing to a single file.
https://winscp.net/eng/docs/ui_transfer_custom

I would like to see an option to autoamtically sync single files, at the moment one can only sync complete folders (keep remote directory up to date). Any chance this will be implemented in future versions? :-)