There seems be be an issue when adding a plugin that has previously been deleted (Oracle 12C database). There is no Boolean type in Oracle 12C but there is an explicit cast in the code at JpaStorage.java:637
see stack trace below:

2017-05-22 11:24:24,719 ERROR [io.apiman.manager.api.jpa.JpaStorage] (default task-44) java.math.BigDecimal cannot be cast to java.lang.Boolean: java.lang.ClassCastException: java.math.BigDecimal cannot be cast to java.lang.Boolean
at io.apiman.manager.api.jpa.JpaStorage.getPlugin(JpaStorage.java:637)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)