Updating Asset Indexes Fails with TemplatesController Not Found Error in Craft Logs Client wants to be able to upload a 90MB .mp4 file to a local asset folder, that understandably throws a 'General error: 2006 MySQL server has gone away'.

I uploaded the file directly to the server via FTP and attempting to re-index the assets throws an unknown error in the CP for each individual asset and crashes the server.

Craft Log reads this error for each asset in the index: 

2019-07-10 13:20:43 [-][-][-][error][yii\web\HttpException:404] yii\web\NotFoundHttpException: Template not found: 403.shtml in "path"/craftcms/cms/src/controllers/TemplatesController.php:87

With either 403.shtml or 500.shtml being the only difference between errors.

There are ~60 assets, all under 8MB, except one other 200MB .mp4 file.

My suspicion is that the server only has 512MB of RAM and it's crashing while attempting to process 300MB worth of data from just two of the 60 assets, as the entire site goes down in the attempt.

PHP Options max_upload memory_limit are 256MB, post_max_size is 128MB, and max_execution and max_input_time time are 240s.
I would check your server logs for any further clues.