In Qt Designer 6.7 the ui file has the same header, "<ui version="4.0">", but this is changed:

"Qt::Vertical" (in older Qt, I think including Qt 6.6.3) to "Qt::Orientation::Vertical" (Qt 6.7 and Qt 6.8). I searched the Qt documentation and Qt::Orientation enum is still Qt::Vertical in Qt 6.7.

Unfortunately, Qt 5.15 has problems when reading these files, and my application looks bad (I have some spacers with vertical orientation there, but Qt 5.15 cannot read this Qt::Orientation::Vertical).