I have been trying for hours now to code a simple AppsScript. When a function on a WebApp server has successfully finished, I want to send the results back to my client (index.html) and call the next function on the server. For some reason, I cannot call a new server function from within my callback function. The first callback is executed - Alert "Callback1" is shown - but the new server function createSheetsLogfile(selectedLabel) is not called after. What am I doing wrong? Is it not possible to call a function on the server from within a callback function?

Thank you very much for your help! Best wishes, Ben

index.html

<button onclick="archiveSBEmails()">Start archiving</button>

<script>
var selectedLabel = "";

function archiveSBEmails(){
    selectedLabel = "Testlabel_1";
    google.script.run.withSuccessHandler(callbackFolderCreated).createFolder(selectedLabel);
 } 
       
function callbackFolderCreated(){
     alert("Callback1");
    google.script.run.withSuccessHandler(callbackLogfileCreated).createSheetsLogfile(selectedLabel);      
}


function callbackLogfileCreated(logfileURL){
    alert("Callback2");
    google.script.run.withSuccessHandler(callbackBackupCompleted).BackupGmailToDrive(selectedLabel);     
}     


function callbackBackupCompleted(){
/...
 }


</script>

code.gs

function doGet() {
  return HtmlService.createHtmlOutputFromFile('index')
            .setSandboxMode(HtmlService.SandboxMode.IFRAME);
}

function createFolder(selectedLabel){  
  //function is called from index.html
  // some code ...
    return true;
}

function createSheetsLogfile(selectedLabel){
  //function is called from index.html
  //some code...
  return logfileURL;
}

function BackupGmailToDrive(selectedLabel){
  //function is called from index.html
  //some code..
  return true;
}