lyx and QT5 blank windows

Sun Apr 26 09:15:18 GMT 2020

I am trying to rebuild lyx with QT5 enabled instead of QT4.
The build is fine and it seems to start fine, but opening new windows 
like "open file" only produces a blank and black window.
At running I see only this output, that is not clear if it is related
QXcbShmImage: shmget() failed (88: Function not implemented) for size 
1407600 (690x510)
QXcbShmImage: shmget() failed (88: Function not implemented) for size 
518976 (318x408)
Gtk-Message: GtkDialog mapped without a transient parent. This is 
discouraged.
QXcbShmImage: shmget() failed (88: Function not implemented) for size 
12000 (100x30)
and at build time there is a new warning coming from the compiler
/usr/include/qt5/QtGui/qtransform.h: In member function ‘QTransform& 
QTransform::operator=(QTransform&&)’:
/usr/include/qt5/QtGui/qtransform.h:81:46: warning: ‘void* memcpy(void*, 
const void*, size_t)’ writing to an object of type ‘class QTransform’ 
with no trivial copy-assignment; use copy-assignment or 
copy-initialization instead [-Wclass-memaccess]
 81 | { memcpy(this, &other, sizeof(QTransform)); return *this; }
 | ^
Anyone had similar problem with other programs ?
Regards
Marco