Sometimes the ISP/router is the cause of this problem due to the firewall you might need to disable the firewall of your router to fix this issue if not then try to use a VPN or if you already using VPN try to turn off VPN and try again.
If it still doesn't work try to change your PC DNS(Google or Cloudflare DNS) and try again.

You have a similar issue on Electrum Github posted years ago you can read them here "The server returned an error when broadcasting the transaction"

If all suggestions do not work you can broadcast the transaction manually to do that you need to copy the raw/hex transaction from your Electrum wallet and broadcast them to any PushTX site listed from the link below

- https://en.bitcoin.it/wiki/Transaction_broadcasting