My squad have been doing a lot of Cloud imports recently and we've run into all sorts of different errors that hide behind that message.
I'm afraid it could be any one of a number of things, and as you need the logs to start diagnosis but don't have access to them on Cloud, the short answer is "we can't even begin to guess".
You should raise it with https://support.atlassian.com/contact as soon as possible. They can at least read the logs and tell you if it's a bug on Cloud, data it can't handle, or a broken export.
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