If I pause media in the Plex Web player for longer than a few seconds, when I unpause it I get the "something went wrong when playing your video" message and this error appears in the console (URL redacted):
[Report Only] Refused to load media from '[my server]' because it violates the following Content Security Policy directive: "default-src 'self'". Note that 'media-src' was not explicitly set, so 'default-src' is used as a fallback.
I am using the unstable release of Chrome on Ubuntu, but this has been happening for a few Chrome versions now and is likely due to an upcoming policy change for Google Chrome (although I'm not sure what). It's been happening on the Canary release for over a month, but I initially thought it was a problem with Windows rather than Plex or Chrome. The change that triggered this appears to have now moved up the release chain to Unstable, so this will likely start affecting the stable release soon.