Are the prayers, supplications, and fasting of one who went to a diviner and then repented accepted? And are all of his deeds invalidated, or only the prayer? And are only the obligatory prayers or all prayers invalidated? And do the deeds that were annulled return after repentance?

Consulting a soothsayer is a grave sin requiring repentance. The statement in the noble Hadith (definition) (his prayer will not be accepted for forty nights) means that no reward will be accrued for it, not that it is invalid and needs to be repeated. This applies to all prayers (obligatory and supererogatory). One who repents from a sin is like one who has no sin. If a servant repents with sincere repentance, the reward for his deeds that were nullified due to the disobedience will be restored to him.