Catalysis of the reaction: 3-O-(N-acetyl-beta-D-glucosaminyl)-L-seryl/L-threonyl- + H2O = L-seryl//L-threonyl- + N-acetyl-D-glucosamine.
-3-O-(N-acetyl-D-glucosaminyl)-L-serine/L-threonine O-N-acetyl-alpha-D-glucosaminase activity