Just put the whole path in "double quote marks".
thanks. OK, now I must confess I don't really know what was going wrong exactly, but I resolved the error.
My main file had a bunch of variables defining each instrument, and a separate file for score layout that placed them. A bunch of those variables weren't yet defined; now that they are, the error no longer appears. I have no idea how that relates to whether the viola part had accidentals on some notes, or whether a rest was beamed, or whatever, but now it works again.
Coding discipline saves the day, I guess.
Thanks for the help. Sorry I couldn't get anything more productive, but I couldn't for the life of me reproduce the error, at any point, using gdb. It only failed in frescobaldi, and (apparently) only because some instruments were still undefined.