Eventually found the cause for my segfault..
I used three global string-vars that got set by some classes and used elsewhere
for context-data.
For some reason the system tried to free their memory twice (found by single-stepping with gdb)
I moved them to their own class and used getter/setters for access and passed a pointer to this
class around, and the segfault went away.
Took days to find & fix...
Lesson learned: try to avoid global variables!
Hope this will help someone with similar symptoms