Sorry that you're unable to export .mp4 videos from Lightroom 7.3.1.
Could you please let us know the size of the video file you're trying to export? Also, please check that Quicktime on your MAC is updated to latest version.
The Video Size: 45 MB
Quicktime Version 10.4 (928.13)
It's the same with Lightroom 7.4
I'm seeing the same behavior in 7.5 Release.
I am trying to export a 28.1MB MOV file to MP4, and it just sits there forever.
I checked the system.log and found the following if this helps...:
Sep 15 23:00:47 Adobe Lightroom Classic: assertion failed: 17G65: libxpc.dylib + 75013 [0BC7AD67-671D-31D4-8B88-C317B8379598]: 0x89
Sep 15 23:00:48 Adobe Lightroom Classic: objc: Class FIFinderSyncExtensionHost is implemented in both /System/Library/PrivateFrameworks/FinderKit.framework/Versions/A/FinderKit (0x7fff98716c90) and /System/Library/PrivateFrameworks/FileProvider.framework/OverrideBundles/FinderSyncCollab orationFileProviderOverride.bundle/Contents/MacOS/FinderSyncCollaborationFileProviderOverr ide (0x141b37cd8). One of the two will be used. Which one is undefined.
Sep 15 23:00:49 Adobe Lightroom Classic: BUG in libdispatch client: kevent[mach_recv] monitored resource vanished before the source cancel handler was invoked