You’re seeing a known server bug.
Do you have Hardware Transcoding enabled?
If so, do you have the error with it turned off?
Dec 31, 2017 00:02:08.768 [0x7f624d3fd700] DEBUG - Downloading document http://127.0.0.1:32400/library/metadata/1?includeBandwidths=1&offset=-1&X-Plex-Session-Identifier=19tllkkgy1rj09426jikl1di
Dec 31, 2017 00:02:08.769 [0x7f624d3fd700] DEBUG - Auth: authenticated user 1 as ackka
Dec 31, 2017 00:02:08.776 [0x7f624d3fd700] DEBUG - We're going to try to auto-select an audio stream for account 1.
Dec 31, 2017 00:02:08.776 [0x7f624d3fd700] DEBUG - Selecting best audio stream for part ID 1 (autoselect: 1 language: en)
Dec 31, 2017 00:02:08.776 [0x7f624d3fd700] DEBUG - Audio Stream: 2060, Subtitle Stream: -1
Dec 31, 2017 00:02:08.777 [0x7f624d3fd700] DEBUG - Codecs: testing h264_qsv (encoder)
Dec 31, 2017 00:02:08.778 [0x7f624d3fd700] DEBUG - Codecs: hardware transcoding: testing API qsv
Dec 31, 2017 00:02:08.778 [0x7f624d3fd700] ERROR - [FFMPEG] - Failed to initialise VAAPI connection: -1 (unknown libva error).
Dec 31, 2017 00:02:08.778 [0x7f624d3fd700] DEBUG - Codecs: hardware transcoding: opening hw device failed - probably not supported by this system, error: Input/output error
Dec 31, 2017 00:02:08.779 [0x7f624d3fd700] DEBUG - Codecs: testing h264_vaapi (encoder)
Dec 31, 2017 00:02:08.779 [0x7f624d3fd700] DEBUG - Codecs: hardware transcoding: testing API vaapi
Dec 31, 2017 00:02:08.779 [0x7f624d3fd700] ERROR - [FFMPEG] - Failed to initialise VAAPI connection: -1 (unknown libva error).
Dec 31, 2017 00:02:08.779 [0x7f624d3fd700] DEBUG - Codecs: hardware transcoding: opening hw device failed - probably not supported by this system, error: Input/output error
Dec 31, 2017 00:02:08.779 [0x7f624d3fd700] DEBUG - Codecs: testing h264_nvenc (encoder)
Dec 31, 2017 00:02:08.779 [0x7f624d3fd700] ERROR - [FFMPEG] - Cannot load libcuda.so.1
Dec 31, 2017 00:02:08.779 [0x7f624d3fd700] WARN - avcodec_open2 returned -1313558101 for encoder 'h264_nvenc'
Dec 31, 2017 00:02:08.779 [0x7f624d3fd700] DEBUG - Scaled up video bitrate to 10080Kbps based on 1.500000x fudge factor.
Dec 31, 2017 00:02:08.780 [0x7f624d3fd700] DEBUG - MDE: Selected protocol dash; container: mp4
Dec 31, 2017 00:02:08.780 [0x7f624d3fd700] DEBUG - MDE: analyzing media item 1
Dec 31, 2017 00:02:08.780 [0x7f624d3fd700] DEBUG - MDE: 20,000 Leagues Under the Sea (1954): Direct Play is disabled
Dec 31, 2017 00:02:08.780 [0x7f624d3fd700] DEBUG - MDE: 20,000 Leagues Under the Sea (1954): media must be transcoded in order to use the dash protocol
Dec 31, 2017 00:02:08.780 [0x7f624d3fd700] DEBUG - MDE: 20,000 Leagues Under the Sea (1954): no direct play video profile exists for http/mkv/mpeg2video
Dec 31, 2017 00:02:08.780 [0x7f624d3fd700] DEBUG - MDE: 20,000 Leagues Under the Sea (1954): no direct play video profile exists for http/mkv/mpeg2video/ac3
Dec 31, 2017 00:02:08.780 [0x7f624d3fd700] DEBUG - MDE: 20,000 Leagues Under the Sea (1954): no remuxable profile found, so video stream will be transcoded
Dec 31, 2017 00:02:08.780 [0x7f624d3fd700] DEBUG - Codecs: testing h264_nvenc (encoder)
Dec 31, 2017 00:02:08.780 [0x7f624d3fd700] ERROR - [FFMPEG] - Cannot load libcuda.so.1
Dec 31, 2017 00:02:08.780 [0x7f624d3fd700] WARN - avcodec_open2 returned -1313558101 for encoder 'h264_nvenc'
Dec 31, 2017 00:02:08.780 [0x7f624d3fd700] DEBUG - Scaled up video bitrate to 10080Kbps based on 1.500000x fudge factor.
Dec 31, 2017 00:02:08.780 [0x7f624d3fd700] DEBUG - MDE: 20,000 Leagues Under the Sea (1954): selected media 0 / 1
Dec 31, 2017 00:02:08.781 [0x7f624d3fd700] DEBUG - Streaming Resource: Adding session 0x7f625ac22720:19tllkkgy1rj09426jikl1di which is using transcoder slot. Used slots is now 1
Dec 31, 2017 00:02:08.781 [0x7f624d3fd700] DEBUG - Streaming Resource: Added session 0x7f625ac22720:19tllkkgy1rj09426jikl1di