Essentials v9 introduced the new Experience gain formula, which gives a minimum of 1 Exp. Version 8 and earlier only have the old formula, which can result in 0 Exp being gained as described.
This is yet another case of the missing Exp values in PSign's pokemon.txt causing problems. He has (finally) uploaded a fixed version of it now, so use that.
As a side note, one would think that if one is using a particular addition (namely PSign's BW pack), then questions about it should go in its thread.
