Cannot read file: bad format

MuseScore 2 can't open scores from MuseScore 3, you get that error only after having ignored the corresponding message, so that much is normal and by design. As to why MuseScore 3 crashes on opening it, there's investigation needed.

Edit: stack trace (in the latest development build):
1 Ms::Drumset::noteHeads drumset.h 74 0xd5a75b
2 Ms::Note::noteHead note.cpp 811 0x871dba
3 Ms::Note::bboxRightPos note.cpp 843 0x871f57
4 Ms::Score::layoutChords2 layout.cpp 614 0x83db20
5 Ms::Score::layoutChords1 layout.cpp 194 0x83c72a
6 Ms::Score::getNextMeasure layout.cpp 2641 0x846566
7 Ms::Score::collectLinearSystem layoutlinear.cpp 179 0x9d0820
8 Ms::Score::layoutLinear layoutlinear.cpp 194 0x9d0896
9 Ms::Score::doLayoutRange layout.cpp 4197 0x84f3ec
10 Ms::Score::update cmd.cpp 220 0x93d74d
11 Ms::readScore file.cpp 2331 0x60a389
12 Ms::MuseScore::readScore file.cpp 341 0x5f8bfe
13 Ms::MuseScore::openScore file.cpp 319 0x5f8ac3
14 Ms::MuseScore::loadFiles file.cpp 299 0x5f87b8
15 Ms::MuseScore::cmd musescore.cpp 5793 0x4c4c58
16 Ms::MuseScore::cmd musescore.cpp 5614 0x4c3ecc
17 Ms::MuseScore::qt_static_metacall moc_musescore.cpp 838 0x72cef3
18 ZN11QMetaObject8activateEP7QObjectiiPPv 0x68c7b9c2
19 ZN12QActionGroup7hoveredEP7QAction 0x25854d75

Something about a whole notedhead in a drumset, pitch 52?

About time to enter this into the issue tracker

See #305293: Crash on load if percussion staff with no drumset uses custom notehead