Just registered to chime in as I happened to be running into this myself and had thought it might be a quirk from compiling/running with MacOSX.  I haven't done much digging into it, and when I get a chance I'll try the debug backtrace mentioned already.  I can also confirm that on my system the problem arises between 4.9.2 and 4.10.0 releases.

In case it might be useful to those more familiar with the code, a few crude tests by way of re-building seem to isolate the problem to setting '--enable-icarus' during configuration.   That is, including all of the non-standalone drivers minus icarus seems to have a functioning write_config on my system.  So do not think this is something to do with all parsing/writing for settings, but may be specific to one or more introduced with icarus support.