What I did : delete any search folder,
'S', change field to 'From', enter 'Luca' and select the option to search in all folders.

then: 
18:22:30 - kmail2(3995) - log_mailcommon: MailCommon::SearchRuleWidget::setRule: ( "\"Subject\" <contains> \"\"" )
18:22:47 - kmail2(3995) - log_kmail: KMail::SearchWindow::doSearch: "{\n    \"limit\": -1,\n    \"negated\": false,\n    \"rel\": 0,\n    \"subTerms\": [\n        {\n            \"negated\": false,\n            \"rel\": 1,\n            \"subTerms\": [\n                {\n                    \"cond\": 5,\n                    \"key\": \"from\",\n                    \"negated\": false,\n                    \"value\": \"Luca\"\n                }\n            ]\n        }\n    ]\n}\n"
18:22:47 - kmail2(3995) - log_kmail: KMail::SearchWindow::doSearch:  create new folder  "Last Search"
18:22:47 - kmail2(3995) -  Akonadi::Job::doHandleResponse: Akonadi::SearchCreateJob(0x27a1950) Unhandled response:  25 "\n{ Response: \"FetchCollections\"\nError Code: \"0\"\nError Msg: \"\"\nID: \"355\"\nName: \"Last Search\"\nParent ID: \"1\"\nRemote ID: \"\"\nRemote Revision: \"\"\nResource: \"akonadi_search_resource\"\nMimetypes: \"(message/rfc822)\"\nStatistics: { Count: \"-1\"\n              Unseen: \"-1\"\n              Size: \"-1\" }\nSearch Query: \"{\n    \"limit\": -1,\n    \"negated\": false,\n    \"rel\": 0,\n    \"subTerms\": [\n        {\n            \"negated\": false,\n            \"rel\": 1,\n            \"subTerms\": [\n                {\n                    \"cond\": 5,\n                    \"key\": \"from\",\n                    \"negated\": false,\n                    \"value\": \"Luca\"\n                }\n            ]\n        }\n    ]\n}\n\"\nSearch Collections: \"QVector(0)\"\nCache Policy: { Inherit: \"true\"\n                Interval: \"-1\"\n                Cache Timeout: \"-1\"\n                Sync on Demand: \"false\"\n                Local Parts: \"()\" }\nAncestors: {  }\nAttributes: \"QMap((AccessRights, luD))\"\nDisplay: \"Undefined\"\nSync: \"Undefined\"\nIndex: \"Undefined\"\nEnabled: \"true\"\nVirtual: \"true\"\nReferenced: \"false\" }"
18:22:47 - kmail2(3995) -  : ASSERT: "mFolder.isValid()" in file /kde/src/5/kdepim/kmail/searchdialog/searchwindow.cpp, line 552

bt:

#4  0x00007ffff56e4e6e in qt_assert(char const*, char const*, int) (assertion=<optimized out>, file=<optimized out>, line=<optimized out>) at global/qglobal.cpp:2966
#5  0x00007ffff7831593 in KMail::SearchWindow::searchDone(KJob*) (this=0x1feecb0, job=0x27a1950) at /kde/src/5/kdepim/kmail/searchdialog/searchwindow.cpp:552
#6  0x00007ffff7838abe in QtPrivate::FunctorCall<QtPrivate::IndexesList<0>, QtPrivate::List<KJob*>, void, void (KMail::SearchWindow::*)(KJob*)>::call(void (KMail::SearchWindow::*)(KJob*), KMail::SearchWindow*, void**) (f=(void (KMail::SearchWindow::*)(KMail::SearchWindow * const, KJob *)) 0x7ffff783106c <KMail::SearchWindow::searchDone(KJob*)>, o=0x1feecb0, arg=0x7fffffffca50)
    at /usr/include/qt5/QtCore/qobjectdefs_impl.h:501
#7  0x00007ffff783877c in QtPrivate::FunctionPointer<void (KMail::SearchWindow::*)(KJob*)>::call<QtPrivate::List<KJob*>, void>(void (KMail::SearchWindow::*)(KJob*), KMail::SearchWindow*, void**) (f=
    (void (KMail::SearchWindow::*)(KMail::SearchWindow * const, KJob *)) 0x7ffff783106c <KMail::SearchWindow::searchDone(KJob*)>, o=0x1feecb0, arg=0x7fffffffca50)
    at /usr/include/qt5/QtCore/qobjectdefs_impl.h:520
#8  0x00007ffff783852d in QtPrivate::QSlotObject<void (KMail::SearchWindow::*)(KJob*), QtPrivate::List<KJob*>, void>::impl(int, QtPrivate::QSlotObjectBase*, QObject*, void**, bool*) (which=1, this_=0x2828dc0, r=0x1feecb0, a=0x7fffffffca50, ret=0x0) at /usr/include/qt5/QtCore/qobject_impl.h:143
#9  0x00007ffff58f9e07 in QMetaObject::activate(QObject*, int, int, void**) (a=0x7fffffffca50, r=0x1feecb0, this=<optimized out>) at ../../src/corelib/kernel/qobject_impl.h:124
#10 0x00007ffff58f9e07 in QMetaObject::activate(QObject*, int, int, void**) (sender=0x27a1950, signalOffset=<optimized out>, local_signal_index=<optimized out>, argv=<optimized out>) at kernel/qobject.cpp:3698
#11 0x00007ffff6f49cdb in KJob::result(KJob*, KJob::QPrivateSignal) (this=0x27a1950, _t1=0x27a1950) at /kde/build/5/kcoreaddons/src/lib/moc_kjob.cpp:555
#12 0x00007ffff6f48731 in KJob::emitResult() (this=0x27a1950) at /kde/src/5/kcoreaddons/src/lib/jobs/kjob.cpp:294
#13 0x00007fffefc1a049 in Akonadi::JobPrivate::delayedEmitResult() (this=0x2827690) at /kde/src/5/kdepimlibs/akonadi/src/core/jobs/job.cpp:153
#14 0x00007fffefc1b111 in Akonadi::Job::qt_static_metacall(QObject*, QMetaObject::Call, int, void**) (_o=0x27a1950, _c=QMetaObject::InvokeMetaMethod, _id=7, _a=0x2826a50)
    at /kde/build/5/kdepimlibs/akonadi/src/core/moc_job.cpp:108
#15 0x00007ffff58fad69 in QObject::event(QEvent*) (this=0x27a1950, e=<optimized out>) at kernel/qobject.cpp:1239
#16 0x00007ffff65af8cc in QApplicationPrivate::notify_helper(QObject*, QEvent*) (this=this@entry=0x672cd0, receiver=receiver@entry=0x27a1950, e=e@entry=0x2826450) at kernel/qapplication.cpp:3717