Is congregational supplication (du'a), especially after the prayer, forbidden, and what is the meaning of the hadith: "Whoever has a need from Allah, let him supplicate for it after every obligatory prayer"?

Collective supplication after the prescribed prayers is an innovated bid'ah unless there is a temporary reason for it and it is not habitual. It is permissible to do it occasionally after supererogatory prayers like Witr, provided it is not taken as a habit. No hadith with the exact wording mentioned has been reported. However, a good (hasan) hadith states that among the times when supplication is answered is following the prescribed prayers, and this includes both before and after the salutation (taslim).