QuickTime won't open MP4 files after updating to Tahoe
mp4 files that opened with Quicktime months ago will no longer open after updating to Tahoe. Is there a fix?
iMac 24″, macOS 26.2
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Not all files with an .mp4 extension necessarily use the same codecs under the hood. QuickTime Player must no longer support either the audio codec, video codec, or container format that your .mp4 files use.
I would recommend installing one of the most useful video tools of all time called "VLC media player" (https://www.videolan.org/vlc/). VLC should not only be able to play it but has menu items that will allow you to determine exactly which codecs and container format are being used.