SOLVED: "playback failed due to fatal player error" Only on webapp

[aeternalis]

Code:

Stream mapping:
Stream #0:0 -> #0:0 (hevc (native) -> av1 (av1_qsv))
Stream #0:1 -> #0:1 (aac (native) -> aac (libfdk_aac))
Press [q] to stop, [?] for help
[av1_qsv @ 0x638820f909c0] This version of runtime doesn't support AV1 encoding

Your GPU doesn't support AV1. In Dashboard->Playback->Transcoding : Disable encoding and decoding in AV1 (these are two separate options).

A very quick test seems to have made this work. Many thanks for the help!