I'm currently trying to automate my series episodes renaming, subtitle fetching and moving via fn:amc and later with filebot node on my Diskstation. Sometimes the episodes are so new, that there is no subtitle available. As far as my tries went, filebot still renames and moves the episodes. Is it ...

volume1/... is relative path starting from your current directory. As your are in /volume1/homes/admin, that explains the weird path /volume1/homes/admin/volume1/video/downloads.
What you want is /volume1/video/downloads