I have seen that a lot of questions are asked about this. Trying to post a #video, and it doesn't work?
First check 3 things. Go to Studio => Host Tools => Files and folders permission.
You should have something like this:

Check that Cron is working, you should have "Last run" <1 minutes:

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On the server side (php.ini) side, these 3 values are important and ideally like this:
max_execution_time = 360
max_input_time = 360
memory_limit = 512M (minimum)
Important: The "upload_max_filesize" and "post_max_size" values of your server (php.ini) will always take precedence over the UNA settings.
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If this works for small videos and not for bigger ones, you may need to increase the "Default cURL timeout" setting.You can find this setting at the bottom of the Studio => Developer page

It currently defaults to 10. It appears to depend on server bandwidth. Test 120 In my case I had to increase it to 300 for a 2 GB video.
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Also, it can be useful to check if FFMPEG is not consuming too many resources, you can see this with the "htop" or"top" command. In my case, "Load average" went up to 8, although that didn't seem to be a problem, I preferred to limit.
This is possible with a utility such as "cpulimite".
Order example:
"cpulimit -e /your_path/plugins/ffmpeg/ffmpeg.exe -l 80"
-l 80 limit to 80% by adding the -v option, you can see it working and testing to find the right limit.

You can then create a.sh script and run it on startup via a cron job with something like:

@reboot /root/scripts/limitesCPU/limitesFFMPEG.sh