I just figure it out that your passphrase is the same password you used to create your opera account. It's so frustrating opera asks for passphrase even if you didn't setup it. So in your case, if you never created any passphrase, this the same password of your opera account. It worked for me. If you ever created any passphrase and forgot it, you can easily reset it. So don't panic with this issue. You can also able to reset your opera account password if you forgot this as well. Reseting passphrase dose not delete any data store in device but do clear all data from Opera server. Means if you newly installed any opera version in any device want to sync your previous data from over Opera account, in general it's not possible.
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