A few notes:
This only seems to affect filenames passed on the command-line, so you
can work around with 'darcs rec -l' or similar to get the files added.
issue2095 is quite related - I suspect the way I fixed that is related
to this problem. One thing to look at (I'll do it when I have a chance)
is whether the GHC version is actually the trigger here, rather than
darcs - GHC 7.0 versus 7.4 would be the relevant thing to check.