It's a website issue.
Refresh your browser, use a different browser, relogin to your Minehut account, or force hibernate & reactivate (MAY cause data loss from up to multiple hours, before attempting save your server, or download worlds important to you)
Try again after each solution and one is bound to work.
But really I recommend not using the addons tab and using spigotMC and downloading the plugin jars straight from there, then upload to your plugins folder and restart.