I then sent these boitcoins to my Poloniex USD generated address.
Please check make sure you've sent the Bitcoins to a Bitcoin (BTC) address. Polo clearly tells in red "please make sure you send only to this address".
I have a hunch that you've sent to an address Polo doesn't handle.
You have a good point there, I now see what I did wrong.
In Poloniex I should had made the transfer to BTC address and not the USDT address. As in the main page where it says "BUY BTC" you must have USDT, I clicked "Deposit USDT" and it gave me a wallet address, I made the transfer to that address.
But shouldn't the transfer from Coinbase not happen due to an incorrect address? And will it be possible to undo the transaction?