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Yeah, I said I guessed a typo was not the reason in this case =)
It is most likely an RPC issue because as I mentioned, this error comes directly from the RPC not finding the account. I cannot reproduce the issue on my side, but ill still try to think of a way to improve the experience at this level.
Thanks for the report!

Good news, I found the issue, it was happening when a whitespace was copy pasted along with the username, causing the blockchain not to recognize it. I fixed it in 2.2.7

Hooray! :)