I just wanted to confirm that I'm able to reproduce the problem on Mac OS X 10.7. Like JF I had to manually patch mktexlsr first (lack of "mktemp --tmpdir" as already reported in "problem (perhaps) with texhash in TL2015 pretest" on the mailing list), but the format generation broke at exactly the same point.
(1) The difference from yesterday in the case of xelatex.fmt: xelatex.ini was updated for LaTeX2e <2015/01/01>, and now unicode-letters.def is used instead of the unicode-letters.tex. There is no problem here in the new LaTeX2e and xelatex.ini. (2) For xetex.fmt, nothing is changed. So it is strange if it was OK yesterday, but it is NG today. Best, Akira
