Found solution. My comment in this topic: https://forums.opera.com/topic/51809/can-t-send-usdt-trc-20-token-or-interact-with-any-tron-smart-contract-dapp-as-opera-shuts-down-each-time?_=1642844837426

@durchmann , Well I resolved this issue for me. Hope this will be helpful for you.
The mnemonic code (12-word backup code) is BIP39 standard (Check this specification for more information https://github.com/bitcoin/bips/blob/master/bip-0039.mediawiki). So there's possibility to restore you private key using that 12-word code outside of opera application. So, what you can do:

• Easiest way and safe way. Use tronlink (official tron extension) https://www.tronlink.org. After extension is set-up click "Import Wallet" and put in the text input your 12-word code. It helped in my case and I just imported my wallet from opera to tronlink.
• More complex and risky solution. I found this tool which converts 12-word backup code into pairs of public-private keys for many blockchains including tron. (https://iancoleman.io/bip39/) You can put your backup phrase into "BIP39 Mnemonic" chose tron blockchain in "bockchain" dropdown and check the list of derived addresses. One of addresses in your list should be yours. This tool by the way has offline version (check Offline Usage section) it runs in your browser so you can be sure your private keys not sent to someone else's server. This is not my software so I can't guarantee it's safe. You use it at your own risk.
  Note : It's Really Risky To Put Your Backup Code Anywhere Outside Of Ooera Crypto Wallet . Don't Do This If You Don't Agree With Potential Money Loss .

@durchmann , I have the same problem with by tron assests. Did you manage to find any solution?

Hi, I just sent tron TRC20 tokens to my crypto wallet address in opera app. But it turned out I can't manage TRC20 assets in opera crypto wallet. Is there any way to get my tokens back. Maybe export private key or sign manually generated transaction using opera crypto wallet?