Problem

Error Details Message
groovy.lang.GroovyRuntimeException: Ambiguous method overloading for method com.exalate.node.hubobject.v1_3.NodeHelper#getIssueType.
Cannot resolve which method to invoke for [class java.lang.String, null] due to overlapping prototypes between:
	[class java.lang.String, interface com.exalate.api.domain.hubobject.v1_2.IHubProject]
	[class java.lang.String, interface com.atlassian.jira.project.Project]

Cause

There is an error in Change processor because of the issue.projectKey variable used in nodeHelper.getIssueType

Solution

In comparison with Create processor where you can use  issue.projectKey in Change Processor you have to use issue.project.key

Check the Sync Rules and correct the code as showed below.

In the  Incoming sync for existing issues(change processor) replace this code 

issue.typeName     = nodeHelper.getIssueType(replica.type?.name, issue.projectKey)?.name ?: "Task"
with this code
issue.typeName = nodeHelper.getIssueType(replica.type?.name, (issue.projectKey ?: issue.project.key))?.name ?: "Task"

For more details check getIssueType nodeHelper method reference.