In case someone else is in the same situation. I just finished porting our plugin to 0.7.0 and the only changes I had to make were in SAMSON::getProxy (the arguments have changed), and the call functions of the proxies (arguments of SBValue type used to be pointers, now they are passed by reference).
We have still to test it to make sure all functionality is working, but so far it seems that was it.
Could you, please, try adding into the SAMSON-Core.sh script for SAMSON-Debug the a path to Qt platforms (/home/bomble/Qt5.9.3/5.9.3/gcc_64/plugins/platforms) to the LD_LIBRARY_PATH environment variable:
which should provide a path to platforms ("xcb" including).
Do you have any Python installed with PyQt5 and their paths set into PATH or LD_LIBRARY_PATH ? The platforms and Qt libs from Python installation may interfere.