This also happened to me. I wanted to transfer 60 Steem from Polo to my SteemIt account, and got that Error... I had to wait 4 months for Polo to reiumburse me back. Then, I tried to do it again, doublechecking my account name and MEMO where all good. However, I got the same error again!
Luckily, I replied back to Polo and in a few hours I have my funds back. Now I think I'll just friecking convert them to BTC and GTFO, leaving them in BTC.
Ok, so it worked for me. My mistake was, ironically, respecting the caps.
My username is 'MrEarthbound', so that's exactly what I typed in the address. This gave me the error.
When I used my username in all lower case 'mrearthbound', the transaction was accepted and I got my Steem in my SteemIt account