What is the reason for seeing disturbing and terrifying dreams, sometimes after Fajr prayer, despite reciting Surat Al-Mulk, Al-Mu'awwidhatayn, Ayat Al-Kursi, the last two verses of Surat Al-Baqarah, and the supplications before sleep, and maintaining the Fajr and Witr prayers? And what is the solution?

Those bad dreams are from Satan. So, if any of you sees a dream that he dislikes, let him seek refuge with Allah from its evil, and let him not mention it to anyone, for it will not harm him. It is prescribed to continue reciting Surah Al-Mulk, the two protective surahs (Al-Falaq and An-Nas), and Ayat al-Kursi before sleeping, as they are a protection against Satan. If Satan still gains control despite that, the flaw is not in the remembrances (dhikr), but rather it might be in the reciter himself or due to an impediment that hinders the effect.