This might be related to a charset problem. Does your username contain special (non-ascii) characters?
If not, it seems that there is no solution at the moment. It might be that boost filesystem is not compatible to your version of MacOS. And boost filesystem exceptions cannot be caught on MacOS - I've reported this problem
http://www.nabble.com/boost-3A-3Afilesystem-exception-not-caught-on-MacOS-X-to13931648.html
but there is no fix, so it is not possible to ignore this error and play without avatars. Sorry.
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