Hi all,

I'm a bit stumped here, so hoping y'all can help me out. I've created a trigger on the Case object to fire the assignment rules and auto-response rules when a case is submitted from our Community (it's an unauthenticated community, so all cases coming in are using the Site Guest User for case submission). The code as it exists below works 100% of the time: 
 

trigger CaseAssignment on Case (after insert) {
    //Variable decleration
    List<Case> caseList = new List<Case>();
    User communityUserObj = new User();
    User adminUserObj = new User();
   
    if(Trigger.isAfter && Trigger.isInsert){
        //Fetching the integration user details
        communityUserObj = [SELECT Id, Name, Alias FROM User where Alias = 'scguest' LIMIT 1];
        adminUserObj = [SELECT Id, Name, Alias FROM User where Alias = 'peter' LIMIT 1];
           
        for (Case caseObj : Trigger.new) {
            if (UserInfo.getUserId()  == communityUserObj.Id || UserInfo.getUserId() == adminUserObj.Id) {
                caseList.add(new Case(id = caseObj.id));
            }
        }
   
        Database.DMLOptions dmo = new Database.DMLOptions();
        dmo.assignmentRuleHeader.useDefaultRule = true;
        Database.update(caseList, dmo);
    }
}

However, when I add the highlighted line of code, the user on the Community page gets an "Insufficient access on cross reference id" error:
 

trigger CaseAssignment on Case (after insert) {
    //Variable decleration
    List<Case> caseList = new List<Case>();
    User communityUserObj = new User();
    User adminUserObj = new User();
   
    if(Trigger.isAfter && Trigger.isInsert){
        //Fetching the integration user details
        communityUserObj = [SELECT Id, Name, Alias FROM User where Alias = 'scguest' LIMIT 1];
        adminUserObj = [SELECT Id, Name, Alias FROM User where Alias = 'peter' LIMIT 1];
           
        for (Case caseObj : Trigger.new) {
            if (UserInfo.getUserId()  == communityUserObj.Id || UserInfo.getUserId() == adminUserObj.Id) {
                caseList.add(new Case(id = caseObj.id));
            }
        }
   
        Database.DMLOptions dmo = new Database.DMLOptions();
        dmo.assignmentRuleHeader.useDefaultRule = true;
        dmo.EmailHeader.triggerAutoResponseRule = true;
        Database.update(caseList, dmo);
    }
}

I've checked all of my permissions for the Guest User and everything is working as it should, but I feel there's a permission or setting I'm missing that would get me around this error. I don't think it's a mixed DML issue, because I'd be getting that error instead, right? Any insight you have would be appreciated. Thanks!