The discord mediatype is validating the wrong url.
URL of webhook: https://discord.com/api/webhooks/........
Mediatype only accepts: https://discordapp.com/api/webhooks

Go to Administration -> Media types and find your discord mediatype. Open it, and next open the "script" part (almost at the bottom of the page)

Find line 23, it should look like:

else if (!params.discord_endpoint.startsWith('https://discordapp.com/api/webhooks')) {

Change it into:

else if (!params.discord_endpoint.startsWith('https://discord.com/api/webhooks')) {

Remove the 'app' from the URL, and the problem is solved