What does "OUTPUT does not recognize tag 'REJECTED'" mean in FME Server?

I have a workspace with a custom transformer that contains GeometryValidator transformer. The transformer should check fme_geometry for basic geometry checks. The workspace works fine in FME Desktop when is run locally. When the workspace is loaded onto FME Server I get the above error message in the logger. Before hitting that transformer the native coordinates of the feature are converted from WKT (the format which enters the workspace) before that transformer (the GeometryValidator) I have also other checks that prevent the process to proceed if the WKT is empty or the area is smaller than a certain threshold, for some reasons when I test it with smaller areas or empty strings the transformer is called and produces the above error. Also, I tried to disable the GeometryValidator transformer but I still get that error, when the data does not even pass by that transformer. If I remove the transformer from the workspace and I upload it to server then everything works fine but I would need to have a further geometry check in place. I am running FME Desktop 2018.1 and FME 2018.0.0.3 Server. Could anyone help? Thanks