Do u think it is illogical to accept that dumbledore had no idea that Riddle diary was a horcrux????

But he did know that the diary was a Horcrux as soon as Harry described him what happened in the Chamber of Secrets, 또는 at least said he did in HBP. :/

he doesn't know that Ginny's book are riddle's diary..
only after harry explain to him he know.