Hallo,
please make sure that there isn't old engine library in Your system (libavastengine-4.so.6).
You can check using cmd version of scanner whether the engine works on Your distribution (the "avast" app instead of "avastgui").
When the application asks license key, then it's started probably first time (in ~/.avast/avastrc there's no line key=BLABLASOMEKEYBLABLA in section [license] yet).
After typing the right key in (wrong keys block the OK button), application starts and engine initialises itself and VPS is expanded, and the main gui window pops up.
Are you able to get to this point?
If so, try to scan another "harmless" location - for example, "/etc" directory (selected folders). This must work with no problem, if not, please tell me more details.
Otherwise, You should be able to find out which particular directory (or particular file) causes the scan to fail so badly using strace. Plese, write more details about this case.
Regards,
PC