I am having the same problem
I added is_array condition in between, and it's ...

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I am having the same problem
I added is_array condition in between, and it's working for me right now

The line 544 now looks like:
if (module_exists('date_repeat') && is_array($form_values[$field_name]) && array_key_exists('rrule', $form_values[$field_name])) {

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