This particular case of amnesia would be due to a bug.
DevForce is stripping off the prefix portion of the method name, in this case "Get", and then expecting the client application to use the remaining portion of the method name, in this case "FilterOperators", in the entity set name argument of the EntityQuery.
You have two options:
1) You can add a QueryAttribute with PreserveName = true to your query method to tell DevForce not to strip off the prefix. You can then continue using the "GetFilterOperators" name on the client.
2) Or you can keep your query method as is, and change your client query like so:
query = new EntityQuery<FilterOperator>("FilterOperators", _mgr);