This just started the last couple days. I updated last night to make sure I was on latest version. I can remotely access my Plex server. Tried through multiple alexa devices. Plex rebooted after update as well. When I tried through the Alexa app, this is what pops up, https://i.imgur.com/LOVI9bm.png. When I try through an Echo device it also adds that maybe the Plex Server is taking to long to respond.
The hotfix went live late last week and there are currently no known issues with remote connections. I checked your user just to confirm and your server isn’t remotely accessible. Please see this support article: https://support.plex.tv/articles/200289506-remote-access/