Ajax error when trying to add another paragraph Drupal 8.9 and paragraph 8.x-1.11
My issue is that Ajax breaks when altering the #validate form. This is an issue for any form! It may have to do with my ajax, but here is my setup.
I have a paragraph type:
contact_information

field_phone_number (telephone from core)
field_type_of_phone (text_plain)

I have a custom module. In this custom module I am creating a form_alter on /user/register
I have:
function telephone_alter_form_user_register_form_alter(&amp;$form, $form_state, $form_id ){
  //$form['#validate'] = array();  //commenting this in or out doesn't help
  $form['#validate'][] = 'telephone_alter_test_validate';
  //$form['field_phone']['widget']['#element_validate'][] = 'telephone_alter_test_validate';
}

function telephone_alter_test_validate($form, &amp;$form_state) {
   echo &quot;hi&quot;; //prints hi, but ajax breaks
}

The validation works, prints out debugging. However, when I try to add another paragraph element, it does nothing. This is for any form. I have tried this on other forms like adding a basic page and trying to call an alter function!
the Ajax error displayed is:
[{&quot;command&quot;:&quot;insert&quot;,&quot;method&quot;:&quot;prepend&quot;,&quot;selector&quot;:null,&quot;data&quot;:&quot;\u003Cdiv data-drupal-messages\u003E\n  \u003Cdiv class=\u0022messages__wrapper\u0022\u003E\n \u003Cdiv class=\u0022alert alert-danger alert-dismissible\u0022 role=\u0022alert\u0022 aria-label=\u0022Error message\u0022\u003E\n \u003Cbutton type=\u0022button\u0022 role=\u0022button\u0022 class=\u0022close\u0022 data-dismiss=\u0022alert\u0022 aria-label=\u0022Close\u0022\u003E\u003Cspan aria-hidden=\u0022true\u0022\u003E\u0026times;\u003C\/span\u003E\u003C\/button\u003E\n \u003Ch2 class=\u0022sr-only\u0022\u003EError message\u003C\/h2\u003E\n \u003Cp\u003EAn unrecoverable error occurred. The uploaded file likely exceeded the maximum file size (40 MB) that this server supports.\u003C\/p\u003E\n \u003C\/div\u003E\n    \u003C\/div\u003E\n\u003C\/div\u003E\n&quot;,&quot;settings&quot;:null}]

The Ajax error is key, because if I create an alter form for any other page, I still get the error.
Any help is great! Thank you! I can provide more info if needed
I had the same issue.
It turned out every time a paragraph is added the validation method runs.
In your case, echo is illegal and it stops the following process. Try replacing it with php error_log or Devel debug functions.