I hate to be redundant but an external search engine can be your best friend.
A search on Google with 'calibre qt.qpa.plugin site:mobileread.com' came up with multiple hits.
See
Calibre 5.0.1 Could not load the Qt platform plugin "xcb" FIXED for one example. Kovid's suggestion of installing libxcb-xinerama0 seems to have worked for several people.