I created a Fluid field and added an existing Grid field and other fields to them. The Grid field contains a Relationships field and a Checkbox field. When saving the entry, I get this error:
Notice Array to string conversion ee/legacy/helpers/custom_field_helper.php, line 40 Severity: E_NOTICE
I checked the file in question and changed this:
$data[$key] = str_replace(array('\\', '|'), array('\\\\', '\|'), $val);
$data[$key] = is_array($val) ? encode_multi_field($val) : str_replace(array('\\', '|'), array('\\\\', '\|'), $val);
…which seems to solve it.