Server Version#: 1.40.0.7998
Player Version#: 4.124.1
This report covers a similar issue in the topic “Plex WebUI Playback Error on refresh” except this error occurs for me consistently while playing content through Plex WebUI and I add content to my server and it [re]scans the library. I was also able to invoke the same error “There was an unexpected error during playback. Please visit our forums if you continue to experience problems” consistently as well by manually choosing “Scan Library Files” from another browser window. I have never experienced this error before until updating to the latest version of Plex server referenced above.