I forgot to mention that I am using Qt 5.5.1, and that  it segfaults while showing the start-up screen. It says Loading... Folios editor.

The problem is with function buildSystemTrayMenu(); in qetapp.cpp file, line 127.

Program received signal SIGSEGV, Segmentation fault.
0x00000000 in ?? ()
(gdb) bt
#0  0x00000000 in ?? ()
0000001  0xb79e0f81 in QMenu::actionEvent(QActionEvent*) () from /usr/lib/libQt5Widgets.so.5
0000002  0xb7892cfa in QWidget::event(QEvent*) () from /usr/lib/libQt5Widgets.so.5
0000003  0xb79e798a in QMenu::event(QEvent*) () from /usr/lib/libQt5Widgets.so.5
0000004  0xb784ac6a in QApplicationPrivate::notify_helper(QObject*, QEvent*) () from /usr/lib/libQt5Widgets.so.5
0000005  0xb7850330 in QApplication::notify(QObject*, QEvent*) () from /usr/lib/libQt5Widgets.so.5
0000006  0xb6d2a8cf in QCoreApplication::notifyInternal(QObject*, QEvent*) () from /usr/lib/libQt5Core.so.5                                                                 
0000007  0xb78891c9 in QWidget::insertAction(QAction*, QAction*) () from /usr/lib/libQt5Widgets.so.5                                                                        
0000008  0xb788933e in QWidget::addAction(QAction*) () from /usr/lib/libQt5Widgets.so.5                                                                                     
0000009  0x081483ec in QETApp::buildSystemTrayMenu (this=0xbffff224) at sources/qetapp.cpp:1529                                                                             
0000010 0x0813fdc5 in QETApp::QETApp (this=0xbffff224, argc=@0xbffff310: 1, argv=0xbffff3a4) at sources/qetapp.cpp:127
0000011 0x08126213 in main (argc=1, argv=0xbffff3a4) at sources/main.cpp:34
(gdb) frame 10
0000010 0x0813fdc5 in QETApp::QETApp (this=0xbffff224, argc=@0xbffff310: 1, argv=0xbffff3a4) at sources/qetapp.cpp:127
warning: Source file is more recent than executable.
127     //      buildSystemTrayMenu();
(gdb)