Notice: Array to string conversion in form_process_checkbox()

I was forced to move to PHP 5.4.9. Could this be the problem?

I confirmed that this error is related to PHP 5.4 I think. There is a backward incompatibility with non-numeric strig offsets of the form

$data['foo']

PHP.net says the following:
■ Non-numeric string offsets - e.g. $a['foo'] where $a is a string - now return false on isset() and true on empty(), and produce a E_WARNING if you try to use them. Offsets of types double, bool and null produce a E_NOTICE. Numeric strings (e.g. $a['2']) still work as before. Note that offsets like '12.3' and '5 foobar' are considered non-numeric and produce a E_WARNING, but are converted to 12 and 5 respectively, for backward compatibility reasons. Note: Following code returns different result. $str='abc';var_dump(isset($str['x'])); // false for PHP 5.4 or later, but true for 5.3 or less

I must move to 5.4.9 soon or find a new host. Is there a workaround?

well it is there in the form.inc file mentioned above. I think it has something to do with indexing into an object that may not be a string. I agree it is obscure.