This bug has been previously reported in the calf bug tracker ( https://github.com/calf-studio-gear/calf/issues/91 ) but after some "investigation" it's seems to be a guitarix issue.
the problem happen when we are using calf reverb from calf 0.0.60 (typically jessie-backports) and when we update calf-plugins from git 72b13fa. guitarix crash when we try to activate the plugin from the rack.
In fact, calf team have reworked their plugins interfaces and some of them have new prameters for visualisation and guitarix doens't seems to like this change.
i found a workaround by deactivating the plugin, delete corresponding file in "$HOME/.config/guitarix/plugins/" and re-enbable the plugin. So i think the problem comes from outdated informations in this file.