IF YOU ARE IN THE RUSSIAN (RU) SERVERS, CLOSE THIS POST. YOU MAY GET BANNED FOR FOLLOWING ANY STEP DESCRIBED HERE. If you have any of those symptoms, as of version 188.8.131.52, there are 2 solution to fix this error (for supported regions). (If you have an older version of BnSBuddy, UPDATE IT)...
The instructions there enable multi-client as well.
Make sure all the plugins and plugin loader are updated.