Resolved: While loading the project, there was an experimental installation of MinGW 64 in the path, which Qt Creator auto-detected as valid 64/32bit toolchain (correct would have been the manually configured standard 32bit Mingw installation). It then set the wrong environment, causing uic to load the 64bit Mingw10.dll, apparently resulting in this hang.
Should 64bit Mingw be auto-detected as 32bit toolchain?

Apparently waits forever for the uic to finish.

Debug log:
00000000 0.00000000 [3444] Warning: No tool chain available for invalid Qt version used in 'Qt4ProjectManager.Target.DesktopTarget'.
00000001 0.19578703 [3444] ctor UiCodeModelSupport for "C:/qt/projects/gitguim/app/mainwindow.ui" "C:/qt/projects/gitguim/app/ui_mainwindow.h"
00000002 0.26582912 [3444] ui*h file not found, or not recent enough, trying to create it on the fly
00000003 0.26627645 [3444] UiCodeModelSupport::runUic "C:/qt/4.8-mingw/qt/bin/uic.exe" on 9300 bytes

0 NtWaitForSingleObject ntdll 0x7782135a
1 WaitForSingleObjectEx KERNELBASE 0x7fefdd210dc
2 QWaitConditionPrivate::wait qwaitcondition_win.cpp 114 0x6b78b18b
3 QWaitCondition::wait qwaitcondition_win.cpp 175 0x6b78b417
4 QWindowsPipeWriter::waitForWrite qwindowspipewriter.cpp 84 0x6b929fe1
5 QProcessPrivate::flushPipeWriter qprocess_win.cpp 778 0x6b91b674
6 QProcessPrivate::closeWriteChannel qprocess.cpp 1130 0x6b8b7a75
7 QProcessPrivate::_q_canWrite qprocess.cpp 1007 0x6b8b7790
8 QProcessPrivate::waitForFinished qprocess_win.cpp 731 0x6b91b457
9 QProcess::waitForFinished qprocess.cpp 1762 0x6b8b8923
10 CppTools::UiCodeModelSupport::runUic uicodecompletionsupport.cpp 150 0x7feeb20efee
11 CppTools::UiCodeModelSupport::init uicodecompletionsupport.cpp 87 0x7feeb20e885
12 CppTools::UiCodeModelSupport::contents uicodecompletionsupport.cpp 115 0x7feeb20ebfd
13 CppTools::Internal::CppModelManager::buildWorkingCopyList cppmodelmanager.cpp 831 0x7feeb1b5871
14 CppTools::Internal::CppModelManager::refreshSourceFiles cppmodelmanager.cpp 878 0x7feeb1b5d6e
15 CppTools::Internal::CppModelManager::updateSourceFiles cppmodelmanager.cpp 848 0x7feeb1b5abf
16 Qt4ProjectManager::Qt4Project::updateCppCodeModel qt4project.cpp 600 0x7fee9226fe9
17 Qt4ProjectManager::Qt4Project::updateCodeModels qt4project.cpp 480 0x7fee9225c1f
18 Qt4ProjectManager::Qt4Project::fromMap qt4project.cpp 399 0x7fee9224e5b
19 ProjectExplorer::Project::restoreSettings project.cpp 226 0x7feeb40f042
20 ProjectExplorer::ProjectExplorerPlugin::openProjects projectexplorer.cpp 1278 0x7feeb35576f
21 ProjectExplorer::Internal::SessionFile::load session.cpp 173 0x7feeb3f594d
22 ProjectExplorer::SessionManager::loadImpl session.cpp 597 0x7feeb3f9c16
23 ProjectExplorer::SessionManager::loadSession session.cpp 1061 0x7feeb3fe8a1
24 ProjectExplorer::ProjectExplorerPlugin::loadSession projectexplorer.cpp 1451 0x7feeb3569a2
25 ProjectExplorer::ProjectExplorerPlugin::qt_static_metacall moc_projectexplorer.cpp 258 0x7feeb50053a
26 QMetaObject::activate qobject.cpp 3549 0x6b98a4d8
27 ProjectExplorer::Internal::ProjectWelcomePage::requestSession moc_projectwelcomepage.cpp 269 0x7feeb5113de
28 ProjectExplorer::Internal::ProjectWelcomePage::qt_static_metacall moc_projectwelcomepage.cpp 209 0x7feeb5111db
29 ProjectExplorer::Internal::ProjectWelcomePage::qt_metacall moc_projectwelcomepage.cpp 252 0x7feeb511327
30 QMetaObject::metacall qmetaobject.cpp 246 0x6b96c744
31 QDeclarativeObjectMethodScriptClass::callMethod qdeclarativeobjectscriptclass.cpp 972 0x6a0014c0
32 QDeclarativeObjectMethodScriptClass::callPrecise qdeclarativeobjectscriptclass.cpp 944 0x6a001205
33 QDeclarativeObjectMethodScriptClass::call qdeclarativeobjectscriptclass.cpp 917 0x6a000ca5
34 QScript::DeclarativeObjectDelegate::call qscriptdeclarativeobject.cpp 154 0x7feec04babb
35 QTJSC::NativeFuncWrapper::operator() CallData.cpp 46 0x7feebf06e97
36 QTJSC::Interpreter::privateExecute Interpreter.cpp 3139 0x7feebec1c6e
37 QTJSC::Interpreter::execute Interpreter.cpp 718 0x7feebeb8126
38 QTJSC::JSFunction::call JSFunction.cpp 122 0x7feebf44913
39 QTJSC::call CallData.cpp 62 0x7feebf06fa7
40 QScriptValue::call qscriptvalue.cpp 1605 0x7feec023d8d
41 QDeclarativeQtScriptExpression::eval qdeclarativeexpression.cpp 518 0x69f0ec2b
42 QDeclarativeQtScriptExpression::scriptValue qdeclarativeexpression.cpp 470 0x69f0e89a
43 QDeclarativeExpressionPrivate::scriptValue qdeclarativeexpression.cpp 653 0x69f0f7ce
44 QDeclarativeExpressionPrivate::value qdeclarativeexpression.cpp 667 0x69f0f8e1
45 QDeclarativeBoundSignal::qt_metacall qdeclarativeboundsignal.cpp 186 0x69f6a544
46 QMetaObject::metacall qmetaobject.cpp 246 0x6b96c744
47 QMetaObject::activate qobject.cpp 3566 0x6b98a5d1
48 QDeclarativeMouseArea::clicked moc_qdeclarativemousearea_p.cpp 524 0x6a0b2f5e
49 QDeclarativeMouseArea::setPressed qdeclarativemousearea.cpp 938 0x69e6f8a3
50 QDeclarativeMouseArea::mouseReleaseEvent qdeclarativemousearea.cpp 588 0x69e6dccc
51 QGraphicsItem::sceneEvent qgraphicsitem.cpp 6739 0x6aea9ccb
52 QDeclarativeItem::sceneEvent qdeclarativeitem.cpp 3013 0x69e5aed2
53 QDeclarativeMouseArea::sceneEvent qdeclarativemousearea.cpp 682 0x69e6e4c0
54 QGraphicsScenePrivate::sendEvent qgraphicsscene.cpp 1218 0x6aeebc42
55 QGraphicsScenePrivate::sendMouseEvent qgraphicsscene.cpp 1292 0x6aeec39b
56 QGraphicsScene::mouseReleaseEvent qgraphicsscene.cpp 4125 0x6aef8bbf
57 QDeclarativeScene::mouseReleaseEvent qdeclarativeview.cpp 128 0x69d44aa8
58 QGraphicsScene::event qgraphicsscene.cpp 3455 0x6aef584b
59 QApplicationPrivate::notify_helper qapplication.cpp 4550 0x6a446896
60 QApplication::notify qapplication.cpp 3932 0x6a443995
61 QCoreApplication::notifyInternal qcoreapplication.cpp 876 0x6b9607ca
62 QCoreApplication::sendSpontaneousEvent qcoreapplication.h 234 0x6ba2cd1c
63 qt_sendSpontaneousEvent qapplication.cpp 5545 0x6a4470fe
64 QGraphicsView::mouseReleaseEvent qgraphicsview.cpp 3339 0x6af3ebad
65 QWidget::event qwidget.cpp 8362 0x6a4e8d64
66 QFrame::event qframe.cpp 557 0x6ab8145f
67 QAbstractScrollArea::viewportEvent qabstractscrollarea.cpp 1043 0x6ac691f1
68 QGraphicsView::viewportEvent qgraphicsview.cpp 2867 0x6af3cbf6
69 QAbstractScrollAreaPrivate::viewportEvent qabstractscrollarea_p.h 100 0x6a41c335
70 QAbstractScrollAreaFilter::eventFilter qabstractscrollarea_p.h 116 0x6ac6b37d
71 QCoreApplicationPrivate::sendThroughObjectEventFilters qcoreapplication.cpp 986 0x6b960a7a
72 QApplicationPrivate::notify_helper qapplication.cpp 4546 0x6a446876
73 QApplication::notify qapplication.cpp 4093 0x6a444409
74 QCoreApplication::notifyInternal qcoreapplication.cpp 876 0x6b9607ca
75 QCoreApplication::sendSpontaneousEvent qcoreapplication.h 234 0x6ba2cd1c
76 QApplicationPrivate::sendMouseEvent qapplication.cpp 3159 0x6a442882
77 QETWidget::translateMouseEvent qapplication_win.cpp 3363 0x6a5345ad
78 QtWndProc qapplication_win.cpp 1696 0x6a52e8b1
79 UserCallWinProcCheckWow USER32 0x77379bd1
80 DispatchMessageWorker USER32 0x773798da
81 QEventDispatcherWin32::processEvents qeventdispatcher_win.cpp 809 0x6b9b995e
82 QGuiEventDispatcherWin32::processEvents qapplication_win.cpp 1202 0x6a52cf45
83 QEventLoop::processEvents qeventloop.cpp 150 0x6b95d7e4
84 QEventLoop::exec qeventloop.cpp 204 0x6b95d9c7
85 QCoreApplication::exec qcoreapplication.cpp 1148 0x6b960e2c
86 QApplication::exec qapplication.cpp 3812 0x6a4435a8
87 main main.cpp 398 0x13ff771ca
88 __tmainCRTStartup crtexe.c 555 0x13ff7c27c
89 mainCRTStartup crtexe.c 371 0x13ff7c0ce
90 BaseThreadInitThunk kernel32 0x7725652d
91 RtlUserThreadStart ntdll 0x777fc521