This paste shows what happens, I am using hypermerchant and essentials eco with the server.
http://paste.md-5.net/guxurotuma.avrasm
I fixed it by removing the essentials userdata folder, however, it did occur again a few hours later.

Could you send me a copy of your Essentials and HyperConomy data? I can look into it. The error message is, unfortunately, not very helpful in this case. Also, are you on the latest version of Essentials and Spigot?