Sponge has done something weird, but i cant seem to find what. Also i saw that the newest version is API 6.0.0, which i haven't made a correct version for. My initial testing for the plugin was done on spongeforge-1.10.2-2092-5.0.0-BETA-1759.jar, with that, everything worked correctly. You could also set in the config to force players to have their effects turned off, that should also fix the error.
EDIT: they have changed their SpongeAPI maven page, and the correct 5.0.0 version isn't even on it somehow.