Happens with current up-to-date KDevelop&KDevPlatform.
This seems to be a regression during the last one/two weeks, as the same session was working fine before.

Backtrace:
#0  0x00007ffff42af425 in __GI_raise (sig=<optimized out>) at ../nptl/sysdeps/unix/sysv/linux/raise.c:64
#1  0x00007ffff42b2b8b in __GI_abort () at abort.c:91
#2  0x00007ffff5a7cef3 in qt_message_output (msgType=QtFatalMsg, 
    buf=0x7fff216d2a28 "ASSERT: \"!DUChain::lock()->currentThreadHasReadLock() && !DUChain::lock()->currentThreadHasWriteLock()\" in file /home/krf/devel/src/kdevplatform/language/highlighting/codehighlighting.cpp, line 160")
    at /home/krf/devel/src/qt/src/corelib/global/qglobal.cpp:2323
#3  0x00007ffff5a7d092 in qt_message(QtMsgType, const char *, typedef __va_list_tag __va_list_tag *) (
    msgType=QtFatalMsg, msg=0x7ffff5c4b4e0 "ASSERT: \"%s\" in file %s, line %d", ap=0x7fff2a1b2f58)
    at /home/krf/devel/src/qt/src/corelib/global/qglobal.cpp:2369
#4  0x00007ffff5a7d85c in qFatal (msg=0x7ffff5c4b4e0 "ASSERT: \"%s\" in file %s, line %d")
    at /home/krf/devel/src/qt/src/corelib/global/qglobal.cpp:2552
#5  0x00007ffff5a7caac in qt_assert (
    assertion=0x7ffff1569650 "!DUChain::lock()->currentThreadHasReadLock() && !DUChain::lock()->currentThreadHasWriteLock()", file=0x7ffff1582808 "/home/krf/devel/src/kdevplatform/language/highlighting/codehighlighting.cpp", line=160)
    at /home/krf/devel/src/qt/src/corelib/global/qglobal.cpp:2018
#6  0x00007ffff155f1fe in KDevelop::CodeHighlighting::highlightDUChain (this=0x27aa970, context=...)
    at /home/krf/devel/src/kdevplatform/language/highlighting/codehighlighting.cpp:160
#7  0x00007fffc839f721 in CPPInternalParseJob::run (this=0x596e100)
    at /home/krf/devel/src/kdevelop/languages/cpp/cppparsejob.cpp:735
#8  0x00007fffec24bf9a in ThreadWeaver::JobRunHelper::runTheJob (this=this@entry=0x7fff2a1b3d50, th=th@entry=
    0x5933df0, job=job@entry=0x596e100) at ../../../threadweaver/Weaver/Job.cpp:106
#9  0x00007fffec24c111 in ThreadWeaver::Job::execute (this=0x596e100, th=0x5933df0)
    at ../../../threadweaver/Weaver/Job.cpp:135
#10 0x00007fffec24d123 in ThreadWeaver::JobCollectionJobRunner::execute (this=0x5920d90, t=0x5933df0)
    at ../../../threadweaver/Weaver/JobCollection.cpp:82
#11 0x00007fffec24b76f in ThreadWeaver::ThreadRunHelper::run (this=this@entry=0x7fff2a1b3dc0, parent=0xb77fc0, 
    th=th@entry=0x5933df0) at ../../../threadweaver/Weaver/Thread.cpp:95
#12 0x00007fffec24b82b in ThreadWeaver::Thread::run (this=0x5933df0) at ../../../threadweaver/Weaver/Thread.cpp:142
#13 0x00007ffff5a8a68e in QThreadPrivate::start (arg=0x5933df0)
    at /home/krf/devel/src/qt/src/corelib/thread/qthread_unix.cpp:338