Magento 1.9.3.9 problem with signature at the last step of OAuth?
I'm using PLAINTEXT method and I do get through all steps of OAuth but the last one (/oauth/token?....). Response is allways "oauth_problem=signature_invalid" despite all previous steps accepted the same PLAINTEXT signature "CONSUMER_SECRET&" (or in URI terms: "CONSUMER_SECRET%26")
Does this specific Magento version have bug or am I missing something?