Stacktrace:

Caused by: java.lang.ClassCastException: ***** cannot be cast to ******
at ConditionEvaluator443abf2927ca4f64a4ad86407ae34799.evaluate(Unknown Source)
at org.drools.rule.constraint.MvelConstraint.evaluate(MvelConstraint.java:200)
at org.drools.rule.constraint.MvelConstraint.isAllowed(MvelConstraint.java:157)
at org.drools.reteoo.FromNode.checkConstraintsAndPropagate(FromNode.java:318)
at org.drools.reteoo.FromNode.assertLeftTuple(FromNode.java:164)
at org.drools.reteoo.CompositeLeftTupleSinkAdapter.doPropagateAssertLeftTuple(CompositeLeftTupleSinkAdapter.java:232)
at org.drools.reteoo.CompositeLeftTupleSinkAdapter.createAndPropagateAssertLeftTuple(CompositeLeftTupleSinkAdapter.java:116)
at org.drools.reteoo.LeftInputAdapterNode.assertObject(LeftInputAdapterNode.java:154)
at org.drools.reteoo.SingleObjectSinkAdapter.propagateAssertObject(SingleObjectSinkAdapter.java:59)

Reason:
ConditionEvaluator seems to be using lazy initializing and then caches generated class to evaluate this expression with another arguments.
Given following:
interface A {
String getString();
}
interface B {
String getString();
}
class X implements B, A {}
class Y implements A{}
rule "test rule"
when:
A(string != null)
then:
end

When rule engine is called for the first time with instance of class X, ConditionEvaluator will bind itself to first found interface implementing method getString(), i.e. interface B.
Thus second call with instance of class Y will cause ClassCastException of casting Y to B.

Solution: force MVEL to bind bytecode generated methods to class/interface declared in the rule explicitly, in our case - to interface A.

Quickfix: use following declaration of class X:
class X implements A, B {}
Because ConditionEvaluator binds to first available interface, it will bind now to correct interface - to interface A.