I don't understand your language, so it's hard to tell what's going on.
But here's a couple problems I can see...

LCDOUT STR cadena,20 'no aparece nada (nothing appears in lcd

should be...

LCDOUT STR cadena\20

And the reason you're missing characters, is probably because the data is received by the interrupt handler, but it's being put into the "buffer" array in the main loop. That should be done in the handler too.
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