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Maybe , this question would better be asked in another forum. If yes, what is the proper one ?

Here

https://factordb.com/index.php?id=1100000000900921624

the factor database seems to have a bug! The number is prime, but it is displayed to be composite.

Does anyone know this site and how to report or remove bugs there ?

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    Maple says it's composite. What makes you think it's prime? – 2017-02-23
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    $$111111111111111111111111111111111111061$$ is prime according to PARI/GP – 2017-02-23
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    The number shown in the link is the ID-number, not the number to be factored. The ID-number is trivially compositive (it is even and greater than $2$) – 2017-02-23

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Let $n = (10^{39}-451) \text{ mod } {\displaystyle{\frac{10^{39}-1}{9}}}$

$\qquad\; = 111111111111111111111111111111111110661$

Then $2^{n-1} \text{ mod }n$ reduces to

$\qquad\;\;\;\;\;\,\, 78402869353071463352746795182243243082$

hence, by Fermat's little Theorem, $n$ is not prime.

So, the "bug" is probably just a typo in how they showed the base-10 expansion of $n$.

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    Or maybe the order of operations is different between the component that finds the decimal expansion and the component that tests primality? The displayed number is `((10^39-451)%(10^39-1))/9`. – 2017-02-27