Thank you! I own a MacBook and am using Safari. Had the same issue. A quick Google search brought me to this page and your advice on using the https://app.plex.tv/ works
It’s a way around this until Plex resolves the problem with an update. Much appreciated. 
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Here is how I fixed the same isssue.
Looking at the address I noticed that secure was set to 0, changing it to 1 solved my problem.
Before:
https://app.plex.tv/desktop?_ga=xxxxxxx&secure=0#
After:
https://app.plex.tv/desktop?_ga=xxxxxxx&secure=1#
Firefox version firefox-68.6.0-1.el8_1.x86_64 on Centos 8.1, Latest Version of Plex Media Server plexmediaserver-1.18.9.2578-513b381af.x86_64, plays fine on TVs and tablets, but not in the Browser.
Tried the various suggestions, within the firefox browser I am not able to view content seeing the error being discussed.
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