#Plex security incident aka data breach confirmed.
"We kindly request that you reset your Plex account password immediately by visiting https://plex.tv/reset - When doing so, there's a checkbox to "Sign out connected devices after password change," which we recommend you enable."
Always enable 2FA.
Full announcement and context (couldn't find it on the Plex website):
https://news.ycombinator.com/item?id=45174684