I've got it now. :)
My first mistake:
PairingAgent pairingAgent;
PairingAgentAdaptor pairingAgentAdaptor(&pairingAgent);
bool registerSuccess = QDBusConnection::systemBus().registerObject(QString(AGENT_PATH), &pairingAgentAdaptor);
It has to be
bool registerSuccess = QDBusConnection::systemBus().registerObject(QString(AGENT_PATH), &pairingAgent); // NEVER the adaptor!!!
My second mistake: I got the wrong interface for org.bluez.Agent1. The right one is:
<!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN" "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
<node>
<interface name="org.bluez.Agent1">
<method name="Release" />
<method name="RequestPinCode">
<arg direction="in" type="o" />
<arg direction="out" type="s" />
</method>
<method name="DisplayPinCode">
<arg direction="in" type="o" />
<arg direction="in" type="s" />
</method>
<method name="RequestPasskey">
<arg direction="in" type="o" />
<arg direction="out" type="u" />
</method>
<method name="DisplayPasskey">
<arg direction="in" type="o" />
<arg direction="in" type="u" />
<arg direction="in" type="q" />
</method>
<method name="RequestConfirmation">
<arg direction="in" type="o" />
<arg direction="in" type="u" />
</method>
<method name="RequestAuthorization">
<arg direction="in" type="o" />
</method>
<method name="AuthorizeService">
<arg direction="in" type="o" />
<arg direction="in" type="s" />
</method>
<method name="Cancel" />
</interface>
</node>