Meanwhile I saw this thread:
https://www.mobileread.com/forums/sho...d.php?t=269806
Vista (some? all?) is missing that function. The workaround is in source and will be part of an upcoming v2.51.0 (next Friday's release).
Possibly a bugfix release v2.50.1 will get released today -- hopefully, since it crashes calibre on a supported platform?
Edit: ninjaed.