I had downloaded both this mod and Witchery, which has Impersonate as a dependent. I couldn't get into a world whatsoever, with the crash logs not really giving me much but naming Impersonate as the culprit. After tinkering around a lot, I managed to remove SMWYG and the issue was resolved. Best I can tell, it has something to do with the way Impersonate handles chat, but I'm not well versed enough to determine anything past a hunch and a circumstantial experiment.