I'm not sure what has changed in my configuration, but KMail now hangs on start. I used it successfully during the previous few days, and I did rebuild KDE PIM in parts a few times, but I didn't update any parts of KDE PIM between the last successful start and the first unsuccessful one.

Below is the backtrace when I stopped KMail in GDB a few seconds after start. It always stops with this same call stack.

---------------
(gdb) run
Starting program: /usr/bin/kmail 
warning: Could not load shared library symbols for linux-vdso.so.1.
Do you need "set solib-search-path" or "set sysroot"?
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".
Qt: gdb: -nograb added to command-line options.
         Use the -dograb option to enforce grabbing.
[New Thread 0x7fffd2bfe700 (LWP 16225)]
QStandardPaths: XDG_RUNTIME_DIR not set, defaulting to '/tmp/runtime-aspotashev'
log_kontactinterface: kontact not running -- start standalone application
log_kmail: 
log_kmail: Delaying instance creation.
log_kmail: Starting up...
"KMail Kernel ETM"
()
connectToServer "/tmp/akonadi-aspotashev.GlVxPd/akonadiserver.socket"
""
connectToServer "/tmp/akonadi-aspotashev.GlVxPd/akonadiserver.socket"
"/subscriber/kmail2_16221_ZlJGyY"
connectToServer "/tmp/akonadi-aspotashev.GlVxPd/akonadiserver.socket"
log_mailtransport: Have SMTP, Sendmail, and 0 Akonadi types.
log_mailtransport: "1853990906"
log_mailtransport: type 0
log_mailtransport: 
Pass a valid window to KWallet::Wallet::openWallet().
log_kmail: 
Pass a valid window to KWallet::Wallet::openWallet().
log_kmail: KMail init with akonadi server state: 2
log_mailcommon: Initialized and looking for specialcollection folders.
"/subscriber/kmail2_16221_IFoW82"
connectToServer "/tmp/akonadi-aspotashev.GlVxPd/akonadiserver.socket"
log_kmail: 
log_kmail: 
"/subscriber/kmail2_16221_3EXfyr"
connectToServer "/tmp/akonadi-aspotashev.GlVxPd/akonadiserver.socket"
log_messagelist: Using model: Akonadi::EntityTreeModel
kf5.kxmlgui: Registering action  "add_addr_book"  under new name  "add_to_existing_contact"
kf5.kxmlgui: Registering action  "openin_addr_book"  under new name  "edit_contact"
"/subscriber/kmail2_16221_h4KtnQ"
connectToServer "/tmp/akonadi-aspotashev.GlVxPd/akonadiserver.socket"
^C
Program received signal SIGINT, Interrupt.
0x00007fffe512111d in connect () from /lib64/libc.so.6
(gdb) bt
#0  0x00007fffe512111d in connect () from /lib64/libc.so.6
#1  0x00007fffdc47d365 in gamin_connect_unix_socket () from /usr/lib64/libfam.so.0
#2  0x00007fffdc47d89a in FAMOpen () from /usr/lib64/libfam.so.0
#3  0x00007fffe790ad11 in KDirWatchPrivate::KDirWatchPrivate() () from /usr/lib64/libKF5CoreAddons.so.5
#4  0x00007fffe790b635 in KDirWatch::KDirWatch(QObject*) () from /usr/lib64/libKF5CoreAddons.so.5
#5  0x00007fffea76f9ce in GrantleeTheme::GrantleeThemeManager::Private::Private(GrantleeTheme::GrantleeThemeManager::Application, QString const&, KActionCollection*, QString const&, GrantleeTheme::GrantleeThemeManager*) () from /usr/lib64/libgrantleetheme.so.5
#6  0x00007fffea76e6d3 in GrantleeTheme::GrantleeThemeManager::GrantleeThemeManager(GrantleeTheme::GrantleeThemeManager::Application, QString const&, KActionCollection*, QString const&, QObject*) () from /usr/lib64/libgrantleetheme.so.5
#7  0x00007ffff59e0c3d in MessageViewer::ViewerPrivate::ViewerPrivate(MessageViewer::Viewer*, QWidget*, KActionCollection*) ()
   from /usr/lib64/libmessageviewer.so.5
#8  0x00007ffff59dd1ef in MessageViewer::Viewer::Viewer(QWidget*, QWidget*, KActionCollection*, QFlags<Qt::WindowType>) ()
   from /usr/lib64/libmessageviewer.so.5
#9  0x00007ffff7744a1f in KMReaderWin::KMReaderWin(QWidget*, QWidget*, KActionCollection*, QFlags<Qt::WindowType>) ()
   from /usr/lib64/libkmailprivate.so.5
#10 0x00007ffff778f2b5 in KMMainWidget::createWidgets() () from /usr/lib64/libkmailprivate.so.5
#11 0x00007ffff778b689 in KMMainWidget::KMMainWidget(QWidget*, KXMLGUIClient*, KActionCollection*, QExplicitlySharedDataPointer<KSharedConfig>) ()
   from /usr/lib64/libkmailprivate.so.5
#12 0x00007ffff7741f65 in KMMainWin::KMMainWin(QWidget*) () from /usr/lib64/libkmailprivate.so.5
#13 0x00007ffff775aadd in KMKernel::openReader(bool) () from /usr/lib64/libkmailprivate.so.5
#14 0x00007ffff77611da in KMKernel::action(bool, bool, QString const&, QString const&, QString const&, QString const&, QString const&, QUrl const&, QList<QUrl> const&, QStringList const&, QString const&, QString const&) () from /usr/lib64/libkmailprivate.so.5
#15 0x00007ffff7759bf7 in KMKernel::handleCommandLine(bool, QStringList const&) () from /usr/lib64/libkmailprivate.so.5
#16 0x000000000040659e in KMailApplication::activate(QStringList const&) ()
#17 0x0000000000406612 in KMailApplication::delayedInstanceCreation(QStringList const&) ()
#18 0x0000000000406826 in main ()
(gdb)

Reproducible: Always