@Daniel -- I'm pretty sure OpenGL isn't a problem here. It was probably just in the middle of painting OpenGL when PortMIDI called exit() which terminated Mixxx. In particular this text:

PortMidi call failed...
  PortMidi: `Invalid MIDI message Data'
type ENTER...

is straight out of the PortMIDI source right before a call to exit() and you can see in the trace for the PortMIDI thread that it has called exit() :

Thread 11 (Thread 0xb0b53b40 (LWP 3945)):
#0  0x00fc5e40 in ?? () from /usr/lib/i386-linux-gnu/libQtOpenGL.so.4
#1  0x033b62fb in __cxa_finalize () from /lib/i386-linux-gnu/libc.so.6
#2  0x00f76fc4 in ?? () from /usr/lib/i386-linux-gnu/libQtOpenGL.so.4
#3  0x01004a1d in _fini () from /usr/lib/i386-linux-gnu/libQtOpenGL.so.4
#4  0x00130ff4 in ?? () from /lib/ld-linux.so.2
#5  0x033b5f51 in ?? () from /lib/i386-linux-gnu/libc.so.6
#6  0x033b5fdd in exit () from /lib/i386-linux-gnu/libc.so.6
#7  0x0016593e in ?? () from /usr/lib/libportmidi.so.0
#8  0x0016599f in ?? () from /usr/lib/libportmidi.so.0
#9  0x00165f41 in Pm_WriteSysEx () from /usr/lib/libportmidi.so.0
#10 0x084339c0 in ?? ()
#11 0x08174668 in ?? ()
#12 0x084b98de in ?? ()
#13 0x084b9bf4 in ?? ()
#14 0x084bd7f3 in ?? ()
#15 0x084d4987 in ?? ()
#16 0x0032bc9d in QMetaObject::metacall(QObject*, QMetaObject::Call, int, void**) () from /usr/lib/i386-linux-gnu/libQtCore.so.4
#17 0x02d26f9f in ?? () from /usr/lib/i386-linux-gnu/libQtScript.so.4
#18 0x02d296b4 in ?? () from /usr/lib/i386-linux-gnu/libQtScript.so.4
#19 0x02d298e9 in ?? () from /usr/lib/i386-linux-gnu/libQtScript.so.4
#20 0x02c1f473 in ?? () from /usr/lib/i386-linux-gnu/libQtScript.so.4
#21 0x02bf3a7f in ?? () from /usr/lib/i386-linux-gnu/libQtScript.so.4
#22 0x078ab390 in ?? ()
#23 0x02baf457 in ?? () from /usr/lib/i386-linux-gnu/libQtScript.so.4
#24 0x02c4bb22 in ?? () from /usr/lib/i386-linux-gnu/libQtScript.so.4
#25 0x02c1f4d9 in ?? () from /usr/lib/i386-linux-gnu/libQtScript.so.4
#26 0x02d1403c in QScriptValue::call(QScriptValue const&, QList<QScriptValue> const&) () from /usr/lib/i386-linux-gnu/libQtScript.so.4
#27 0x0817ce68 in ?? ()
#28 0x081820d5 in ?? ()
#29 0x0817638e in ?? ()
#30 0x081b1ff9 in ?? ()
#31 0x0819105c in ?? ()
#32 0x084bcce1 in ?? ()
#33 0x00336c01 in QMetaCallEvent::placeMetaCall(QObject*) ()
   from /usr/lib/i386-linux-gnu/libQtCore.so.4
#34 0x0033fc7b in QObject::event(QEvent*) ()
   from /usr/lib/i386-linux-gnu/libQtCore.so.4
#35 0x005bfed4 in QApplicationPrivate::notify_helper(QObject*, QEvent*) ()
   from /usr/lib/i386-linux-gnu/libQtGui.so.4
#36 0x005c530d in QApplication::notify(QObject*, QEvent*) ()
   from /usr/lib/i386-linux-gnu/libQtGui.so.4
#37 0x0032497e in QCoreApplication::notifyInternal(QObject*, QEvent*) ()
   from /usr/lib/i386-linux-gnu/libQtCore.so.4
#38 0x00328ad8 in QCoreApplicationPrivate::sendPostedEvents(QObject*, int, QThreadData*) () from /usr/lib/i386-linux-gnu/libQtCore.so.4
#39 0x00328e0c in QCoreApplication::sendPostedEvents(QObject*, int) ()
   from /usr/lib/i386-linux-gnu/libQtCore.so.4
#40 0x00357494 in ?? () from /usr/lib/i386-linux-gnu/libQtCore.so.4
#41 0x036c8d86 in g_main_context_dispatch ()
   from /lib/i386-linux-gnu/libglib-2.0.so.0
#42 0x036c9125 in ?? () from /lib/i386-linux-gnu/libglib-2.0.so.0
#43 0x036c9201 in g_main_context_iteration ()
   from /lib/i386-linux-gnu/libglib-2.0.so.0
#44 0x00357887 in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () from /usr/lib/i386-linux-gnu/libQtCore.so.4
#45 0x0032350d in QEventLoop::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () from /usr/lib/i386-linux-gnu/libQtCore.so.4
#46 0x003237a9 in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) ()
   from /usr/lib/i386-linux-gnu/libQtCore.so.4
#47 0x0020c94c in QThread::exec() ()
   from /usr/lib/i386-linux-gnu/libQtCore.so.4
#48 0x0020ca3b in QThread::run() () from /usr/lib/i386-linux-gnu/libQtCore.so.4
#49 0x0020fde0 in ?? () from /usr/lib/i386-linux-gnu/libQtCore.so.4
#50 0x03076d4c in start_thread () from /lib/i386-linux-gnu/libpthread.so.0
#51 0x03471d3e in clone () from /lib/i386-linux-gnu/libc.so.6