type
   TiboqThread = class( TThread )
   private
      FQuery    : TiboQuery;
      FErrorCode : integer;
      FErrorText : string;
   public
      constructor Create( AQuery : TiboQuery );
      destructor Destroy; override;
      procedure  Execute; override;
      procedure  ClearError;
      procedure  SendError;
   end; { type }

constructor TiboQThread.Create( aQuery : TiboQuery );
begin
   inherited Create( True );
   FQuery := aQuery;
end;

procedure TiboQThread.ClearError;
begin
   LastErrCode := 0;
   LastErrText := '';
end;

procedure TiboQThread.SendError;
begin
   LastErrCode := FErrorCode;
   LastErrText := FErrorText;
end;

procedure TiboQThread.Execute;
var i : integer;
begin
   inherited;
   try
      Synchronize( ClearError );
      FQuery.Open;
   except
      on E : Exception do begin
         FErrorCode := 4313; // ERROR_DATABASE_FAILURE;
         FErrorText := 'Query-Thread:' + E.ClassName + ' - ' + E.Message;
         Synchronize( SendError );
      end;
   end;
end;

destructor TiboQThread.Destroy;
begin
   FQuery := nil;
   inherited;
end;

// Main
// ...
IBQ1.Active := False;
IBQ1.SQL.Text := tmpSql;
// IBQ1.Active := True;
// Try it via Thread
tQ := TiboqThread.create( IBQ1 );
tQ.FreeOnTerminate := False;
tQ.Resume;
while ( WaitForSingleObject( tQ.Handle, 100 ) = WAIT_TIMEOUT ) and
      ( A_Halt.Tag = 0 )
do begin
   Application.ProcessMessages;
end;
if LastErrCode = 0 then begin
   // Alles OK
end else begin
   FehlerAusgabe1( LastErrText );
end;
tQ.Free;
// ...

exception class  : EOSError
exception message : System Error. Code: 1400. Ungültiges Fensterhandle.

main thread ($8b0):
0045ff80 +070 TEST.exe SysUtils                       RaiseLastOSError
0045ff09 +005 TEST.exe SysUtils                       RaiseLastOSError
004bb551 +031 TEST.exe Controls                       TWinControl.DestroyWindowHandle
004b9e0b +05f TEST.exe Controls                       TWinControl.Destroy
004c0ba5 +01d TEST.exe Controls                       TCustomControl.Destroy
0054b8e4 +020 TEST.exe cxControls            1987  +2 TcxSizeGrip.Destroy
00404710 +008 TEST.exe System                  48  +0 TObject.Free
00460630 +008 TEST.exe SysUtils                       FreeAndNil
0054c60d +021 TEST.exe cxControls            2507  +5 TcxControl.DestroyScrollBars
0054bc65 +019 TEST.exe cxControls            2127  +2 TcxControl.Destroy
0070423c +01c TEST.exe cxGridCustomView      4296  +3 TcxGridSite.Destroy
00404710 +008 TEST.exe System                  48  +0 TObject.Free
00460630 +008 TEST.exe SysUtils                       FreeAndNil
00706c6e +006 TEST.exe cxGridCustomView      6071  +1 TcxCustomGridViewInfo.DestroySite
00706a4c +010 TEST.exe cxGridCustomView      5975  +1 TcxCustomGridViewInfo.Destroy
006f2b24 +020 TEST.exe cxGridCustomTableView 12424  +2 TcxCustomGridTableViewInfo.Destroy