The issue arises from the implementation of "Hotspotting", a concept of switching the WiFi state (STA to AP, or visa versa) of (mobile) nodes to connect and exchange data opportunistically.
Node A (station) that has just sent an association request to Node B (AP/hotspot), decides to become an AP/hotspot for other nodes (e.g. because Node B was busy and the association response is delayed).
Node B replies with an association response that is received by Node A and causes the simulation to crash.

That is because (in ApWifiMac::Receive) the case of an association response is not considered.
The received WifiMacHeader is:
- a Management header,
- addressed to this node, but
- neither an association request,
- nor a disassociation.
Hence, RegularWifiMac::Receive is called. In this method arises the
NS_FATAL_ERROR ("Don't know how to handle frame (type=" << hdr->GetType ());
(where hdr->GetType() is WIFI_MAC_MGT_ASSOCIATION_RESPONSE).

To continue my project I chose a workaround. I modified ApWifiMac::Receive so that association responses are dropped:

    // below if (hdr->IsDisassociation ()){...}
    else if (hdr->IsAssocResp())
    {
      NotifyRxDrop (packet);
      return;
    }

Although an access point usually not receives association responses, it should remain functional if this happens.