wp_remote_post rabobank omnikassa (version2) – error 5001

turns out (in case it’s helpful to someone else) that

$omnikassaOrder['signature'] = hash_hmac('sha512', json_encode(implode(',',$toHash)), base64_decode($SigningKey));

should have been simply the following (without json_encode )

$omnikassaOrder['signature'] = hash_hmac('sha512', implode(',',$toHash), base64_decode($SigningKey));