Displays phosphatase activity for serine/threonine residues, and dephosphorylates and activates Pk92B kinase. Has apparently no phosphoglycerate mutase activity. H2O + O-phospho-L-seryl-[protein] = L-seryl-[protein] + phosphate H2O + O-phospho-L-threonyl-[protein] = L-threonyl-[protein] + phosphate Interacts with Pk92B/ASK1. Belongs to the phosphoglycerate mutase family. BPG-dependent PGAM subfamily.