Qtractor 0.5.7 crashes when adding a plugin and having the Plugin window to scan the different plug-in types. I've added a gdb backtrace. This is what I did:
create a new audio track
open the mixer
add a plugin
scan for plugins (LADSPA, DSSI, VST and LV2)
* scan for (any) plugins
I've seen this behavior with earlier svn revisions too.