I am having this exact same issue with a Rest Api in C#. All calls authenticate properly until including something such as an emoji or a non-ascii character. Below is an example (Top lines represent the test’s from Twitters Oauth Tester). The post status is status=æ http://something.com etc.
Signature base string POST&https%3A%2F%2Fapi.twitter.com%2F1.1%2Fstatuses%2Fupdate.json&oauth_consumer_key%3Doauthconsumerkey%26oauth_nonce%3D7b1129a8aff79764ca16e7841d802a11%26oauth_signature_method%3DHMAC-SHA1%26oauth_timestamp%3D1446137121%26oauth_token%3D3728392453-9rka5K8R9eN4Hinh09ocY0z5q0WRaqen79J720w%26oauth_version%3D1.0%26status%3D%25C3%25A6%2520https%253A%252F%252Fdev.app.com%252Fposts%252F00059382-dc28-4d7a-85e9-2de4edc68dd2
Signature base string POST&https%3A%2F%2Fapi.twitter.com%2F1.1%2Fstatuses%2Fupdate.json&oauth_consumer_key%3Doauthconsumerkey%26oauth_nonce%3DI7OgYwsclSet2CJ0nvBfZpB1F9AjCKjY%26oauth_signature_method%3DHMAC-SHA1%26oauth_timestamp%3D1446159566%26oauth_token%3D3728392453-9rka5K8R9eN4Hinh09ocY0z5q0WRaqen79J720w%26oauth_version%3D1.0%26status%3D%25C3%25A6%2520https%253A%252F%252Fdev.app.com%252Fposts%252Fd5904487-d0c0-407d-9338-e4de3b169f9e
Authorization: OAuth oauth_consumer_key="oauthconsumerkey", oauth_nonce="7b1129a8aff79764ca16e7841d802a11", oauth_signature="MkLXuCOxX8%2BE9kAHeEEKGP2o2JE%3D", oauth_signature_method="HMAC-SHA1", oauth_timestamp="1446137121", oauth_token="3728392453-9rka5K8R9eN4Hinh09ocY0z5q0WRaqen79J720w", oauth_version="1.0"
Authorization: OAuth oauth_consumer_key=\"oauthconsumerkey\", oauth_nonce=\"I7OgYwsclSet2CJ0nvBfZpB1F9AjCKjY\", oauth_signature=\"wPS4zalY3jSiRcq%2BOY4zhrvZjZY%3D\", oauth_signature_method=\"HMAC-SHA1\", oauth_timestamp=\"1446159566\", oauth_token=\"3728392453-9rka5K8R9eN4Hinh09ocY0z5q0WRaqen79J720w\", oauth_version=\"1.0\"
curl --request 'POST' 'https://api.twitter.com/1.1/statuses/update.json' --data 'status=%C3%A6+https%3A%2F%2Fdev.app.com%2Fposts%2F00059382-dc28-4d7a-85e9-2de4edc68dd2' --header 'Authorization: OAuth oauth_consumer_key="oauthconsumerkey", oauth_nonce="7b1129a8aff79764ca16e7841d802a11", oauth_signature="MkLXuCOxX8%2BE9kAHeEEKGP2o2JE%3D", oauth_signature_method="HMAC-SHA1", oauth_timestamp="1446137121", oauth_token="3728392453-9rka5K8R9eN4Hinh09ocY0z5q0WRaqen79J720w", oauth_version="1.0"' --verbose
curl -v -X POST -H \"Content-Type: application/x-www-form-urlencoded\" -H \"Accept-Language: en-us,en;q=0.5\" -H \"Accept-Encoding: gzip,deflate\" -H \"Accept-Charset: utf-8;q=0.7,*;q=0.7\" -H \"Authorization: OAuth oauth_consumer_key=\"oauthconsumerkey\", oauth_nonce=\"I7OgYwsclSet2CJ0nvBfZpB1F9AjCKjY\", oauth_signature=\"wPS4zalY3jSiRcq%2BOY4zhrvZjZY%3D\", oauth_signature_method=\"HMAC-SHA1\", oauth_timestamp=\"1446159566\", oauth_token=\"3728392453-9rka5K8R9eN4Hinh09ocY0z5q0WRaqen79J720w\", oauth_version=\"1.0\"\" https://api.twitter.com/1.1/statuses/update.json?status=%C3%A6%20https%3A%2F%2Fdev.app.com%2Fposts%2Fd5904487-d0c0-407d-9338-e4de3b169f9e%2Fuid%2F227951"
@episod can you provide any insight to this?