I had this problem before and identified that the visual composer was disabled, enabled it and the backend worked again.
Right now I repeated the problem with another site and reviewing I did not have the visual composer so I installed it and I already have the backend editor back.
as a note I tell you that I update to wordpress 5.6 and php 7.2 is working well for me.
with php 7.3 certain wpbakery blocks don’t work for me.
