Is the marriage of the questioner to a Dutch Christian woman, which took place without a guardian, considered invalid and forbidden for the past two years, even after it was officially documented in Holland?

The guardian (wali) is a condition for marriage according to the most preponderant opinion, based on the Prophet's (peace be upon him) saying: "There is no marriage without a guardian." If a Muslim marries a dhimmi woman, her disbelieving guardian may marry her off to him. If your wife has a guardian, you must renew the marriage contract because the previous contract is invalid according to the majority of scholars. Otherwise, the competent authority for marriage will act as the guardian, with the presence of two Muslim witnesses. As for your past marriage, if you were following an opinion that permits a mature woman to marry herself without a guardian, then there is no sin upon you.