Okay,
I've worked on this issue a bit myself, debugging the code in main.inc.php, I looked at all the conversions being attempted, and got this list (trimmed):
UTF-8 -> UTF7-IMAP
UTF-8 -> UTF7-IMAP
UTF-8 -> UTF7-IMAP
UTF-8 -> UTF7-IMAP
UTF-8 -> ISO-8859-1
UTF-8 -> ISO-8859-1
UTF-8 -> ANSI
UTF-8 -> ISO-8859-1
UTF-8 -> US-ASCII
UTF-8 -> ISO-8859-1
UTF-8 -> US-ASCII
UTF-8 -> ISO-8859-1
UTF-8 -> ISO-8859-1
UTF-8 -> ISO-8859-1
The warning was only being raised once, so I guessed that mb_convert_encoding() doesn't know what ANSI means, I put in an alternate case structure to handle that case and I don't get the error anymore. This is a.. bad solution, presently I'm returning an empty string for an ANSI string with is sub-optimal. I'll ask someone sooner or later what the best way to convert those is.