When a value is deleted from "Extra length on restart" under Retraction section, saving the configuration makes the Slic3r to crash immediately and unable to start properly.
Manually fixing the .ini file with a value ( e.g. 0 ) makes it normal again and start properly.
Windows platform. Slic3r 0.9.8
Not a terrible bug, but worth fixing. Spent hour looking for the problem. Just to save some time for other users.
Yes, this is #611