Error: Cannot create references to/from string offsets in drupal_array_set_nested_value() (line 6794 of /home/www/html/<studyname>/includes/common.inc).

This is a PHP error stating that you are trying to create a reference to a string offset, which is not allowed in PHP.
The following function (includes/common.inc) caused the error:

function drupal_array_set_nested_value(array &$array, array $parents, $value, $force = FALSE) {
  $ref = &$array;
  foreach ($parents as $parent) {
    // PHP auto-creates container arrays and NULL entries without error if $ref
    // is NULL, but throws an error if $ref is set, but not an array.
    if ($force && isset($ref) && !is_array($ref)) {
      $ref = array();
    }
    $ref = &$ref[$parent];
  }
  $ref = $value;
}

you have to check if $ref is an array and if its initialized already (if not, the function should exit immediately)

try this:

function drupal_array_set_nested_value(array &$array, array $parents, $value, $force = FALSE) {
  $ref = &$array;
  foreach ($parents as $parent) {
    if (!isset($ref) || !is_array($ref)) {
      if ($force) {
        $ref = array();
      }
      else {
        return;
      }
    }
    $ref = &$ref[$parent];
  }
  $ref = $value;
}

it worked for me.

Hi, I have created an issue https://www.drupal.org/project/drupal/issues/3458997 🐛 TypeError: Cannot access offset of type string on string in drupal_array_set_nested_value() (line 6955 of /includes/common.inc). Active , because the search didn't show this one.

I managed to fix this error with this patch.