Is the marriage of a Muslim man to a woman who has recently converted to Islam, and then it becomes clear that she follows an innovated sect, considered null and void, and what are the legal implications of that?

We congratulate the questioner on the blessing of Islam and on following the methodology of Ahl al-Sunnah (definition) wal-Jama'ah. Regarding a man marrying someone who differs in belief, his marriage is valid unless her belief takes her out of the fold of Islam, in which case the marriage must be annulled. The statement of Shaykh al-Islam Ibn Taymiyyah that the child is attributed to his lineage and that inheritance is established in a corrupt marriage believed to be valid does not mean that the marriage itself is valid; rather, both are established merely by the belief [in its validity].