Windows Defender says Plex Media Server 1.40.1.8227 contains a virus

Which model & DSM 6 or 7?

It is a false positive. Make sure your virus definitions are up to date.

I’ve 1.407.426.0 on my PC, downloaded 1.40.1.8227 Intel 64-bit, DSM7, and it was not flagged by Defender.