First of all getting 18 XEM and then 0 is not weird. The reward depends on transaction fees. If there are no transactions in the block you harvested you won’t get a reward.
Secondly I’m guessing you already/still are harvesting. Either you activated delegated harvesting and are trying to activate local harvesting or the other way around. Or you have that NIS set to auto-harvest and are trying to activate it again. I guess there are a couple ways to produce the error but it all comes down to someone is already harvesting on that NIS and you are trying to harvest there again. Is that a remote NIS ? The default Limit of harvesters for any given NIS is 1 which is why you get the error. You can raise it in the config file the location of which eludes me right now.
The UX of that particular part of NCC is terrible so I totally understand why you ran into that error. I understand it’s being worked on though.