Hi Oyvind,

The code is rectified and verified it at my end and it works as expected. This should work fine for you without any errors. Also, since the records are shared using a Salesforce to Salesforce connection, they should be done when the DML operations successfully commit their values to the database or else it might result in inconsistency. Hence I have changed the trigger to fire on After Insert and After Update events.

Trigger AutoforwardOpp on Opportunity(after insert, after update)
{
    List <PartnerNetworkRecordConnection> prncList;
    List <PartnerNetworkConnection> connectionList;
    Map <ID, PartnerNetworkConnection> connMap;
    Map <ID, ID> oppIdvsAccountIdMap;
    Map<ID, Account> accountWithContactMap;
    
    ID cid;
    String status;
    String connName;

    if(Trigger.isAfter && (Trigger.isInsert || Trigger.isUpdate)){
            
        connectionList = new List<PartnerNetworkConnection>();
        prncList = new List<PartnerNetworkRecordConnection>();
        
        //This would ideally return multiple active Connections if they exist hence it is best you use a 
        //criteria to ensure only the appropriate connection record is returned. 
        connMap = new Map<ID, PartnerNetworkConnection>(
            [Select ID, ConnectionStatus, ConnectionName 
             From PartnerNetworkConnection 
             Where ConnectionStatus = 'Accepted']);
        
        //get connection details        
        for(ID connId :connMap.keySet()){
            cid = connMap.get(connId).ID;
            status = connMap.get(connId).ConnectionStatus;
            connName = connMap.get(connId).ConnectionName;
        }
        
        //Populate a map of Opp Ids and associated Account Ids
        oppIdvsAccountIdMap = new Map<ID, ID>();
        for(Opportunity oppRecord :Trigger.new){
        
            if(oppRecord.Department__c == 'US'){
                    oppIdvsAccountIdMap.put(oppRecord.ID, oppRecord.Account.ID);
            }
        }
        
        //Get associated Accounts and Contacts for every US opportunity if they exist
        if(oppIdvsAccountIdMap.keySet().size() > 0){
        
            accountWithContactMap = new Map<ID, Account>(
                [Select ID, Name,
                    (Select ID, Name From Account.Contacts)
                 From Account
                 Where ID IN :oppIdvsAccountIdMap.values()]);

            //Create PartnerNetworkRecordConnections for sharing the records
            for(ID oppId : oppIdvsAccountIdMap.keySet()){
            
                ID accountId = oppIdvsAccountIdMap.get(oppId);
                
                //Share Opportunity
                prncList.add(new PartnerNetworkRecordConnection(
                    ConnectionId = cid,
                    LocalRecordId = oppId,
                    SendClosedTasks = true,
                    SendOpenTasks = true,
                    SendEmails = true));
                
                //Share Account
                if(oppIdvsAccountIdMap.get(oppId) != null){
                
                    prncList.add(new PartnerNetworkRecordConnection(
                        ConnectionId = cid,
                        LocalRecordId = accountId,
                        SendClosedTasks = true,
                        SendOpenTasks = true,
                        SendEmails = true));
                }
                
                //Share associated Contacts
                if(accountWithContactMap.get(accountId).Contacts != null){
                
                    for(Contact contact :accountWithContactMap.get(accountId).Contacts){
                    
                        prncList.add(new PartnerNetworkRecordConnection(
                            ConnectionId = cid,
                            LocalRecordId = contact.ID,
                            SendClosedTasks = true,
                            SendOpenTasks = true,
                            SendEmails = true));
                    }
                }
            }//for
            
            //Insert record conneections
            if(!prncList.isEmpty()){
            
                try{
                    insert prncList;
                }
                catch(System.Dmlexception dmlExceptionInstance){
                    System.debug('Record Share Error:' + dmlExceptionInstance.getMessage());
                }
            }
        }//if
    }
}

I hope you find the above solution helpful. If it does, please mark as Best Answer to help others too.

Thanks and Regards,
Deepali Kulshrestha