java.lang.ClassCastException: java.lang.String cannot be cast to com.atlassian.jira.user.Application

[Yogesh Mude]

import com.atlassian.jira.user.ApplicationUser;
import com.atlassian.jira.component.ComponentAccessor;
import com.atlassian.jira.event.type.EventDispatchOption
import com.atlassian.jira.issue.MutableIssue
import com.atlassian.jira.issue.fields.CustomField

def currentUser = ComponentAccessor.getJiraAuthenticationContext().getLoggedInUser().getName()
ApplicationUser systemUser = ComponentAccessor.getUserManager().getUserByKey("system");
def issueManager = ComponentAccessor.getIssueManager()
CustomField approversCF = ComponentAccessor.getCustomFieldManager().getCustomFieldObject(11718) // Approvers custom field 
def approversCFValue = issue.getCustomFieldValue(approversCF)
def approvers = []

approversCFValue.each { appUser ->
approvers.add(appUser.getName())
}
log.info "Approvers : " +approvers
if(!approvers.isEmpty()){
if(currentUser in approvers){
approvers.remove(currentUser)
}
issue.setCustomFieldValue(approversCF, approvers)
}
issueManager.updateIssue(systemUser, issue, EventDispatchOption.ISSUE_UPDATED, false)
log.info "after removing approvers : " +approvers

[Yogesh Mude]

[Radek Dostál]

def currentUser = ComponentAccessur.getJiraAuthenticationContext().getLoggedInUser().getName()

So you're declaring the currentUser to be String (username). And later on trying to work with it as if it was an ApplicationUser object in

if (currentUser in approvers)
  approvers.remove(currentUser) 

[Yogesh Mude]

Thank you @Radek Dostál for your response.