The problem will actually be an old class version being called in the template override.
I've just had a similar issue:
AcyMailing\Classes\FieldClass::displayField(): Argument #3 ($valuesArray) must be of type array, string given
Prior to 10.6.7 the displayField class that would have loaded when you created the template override had a $size argument in position three. Looking at files in the component I can see that the newer version of the class no longer requires the $size argument.
So change your template override (in my case it was an issue in /html/com_acym/frontusers/profile.php) so that the offending line goes from:
$fielddisplay = $data['fieldClass']->displayField($field, $field->default_value, $size, $valuesArray, true, true, $data['user']);
to this without the $size value:
$fielddisplay = $data['fieldClass']->displayField($field, $field->default_value, $valuesArray, true, true, $data['user']);
This is better than just disabling the plugin or deleting the override, as chances are if the creation of the override was done for a particular reason, then the reason would be taken away by changing the override.
I'm not sure where there's further documentation on the AcyMailing\Classes\FieldClass::displayField() class (if any) and a search for displayfield in the forum only shows this thread and one other.
