>

>INSERT INTO base1.dbo.PRESTAMOS
>(idprestamo,idcliente,fecha, fechap1, fechav, duracion, interes, comision, monto, balance,idtipo, fecha_pago, 
> cuota,idpago)
>SELECT CAST(RIGHT([Código Préstamo],8) AS INT) AS IDPRESTAMO,CAST(RIGHT(CodCliente,8) AS INT) AS IDCLIENTE,
>FECHA,Inicio AS FECHAP1,Vence AS FECHAV,Cuotas AS DURACION,cast(Tasa as numeric(12,2))*12 AS INTERES,0 AS COMISION,
>Monto,case when Balance>0 then monto else 0 end as balance,6 as idtipo,
>convert(varchar(10),CONVERT(datetime,ultpago,105),111) AS FECHA_PAGO,[Monto Cuotas]AS Cuota,
>IDPAGO=CASE WHEN periodopago='D' THEN 1 ELSE 
>CASE WHEN periodopago='S' THEN 2 ELSE
>CASE WHEN periodopago='Q' THEN 3 ELSE 4 END END END FROM Préstamo
>

I suggest to make this minor change close to the end:

CASE   periodopago WHEN 'D' THEN 1  WHEN 'S' THEN 2 WHEN Q' THEN 3 ELSE 4 END AS IDPAGO

to simplify the expression a bit.

In any case, the problem most likely lies in the trigger's code - can you post it?