Sorry, if it turns out as a stupid mistake, but I am at the end of my own knowledge to solve this issue on my MediaWiki 1.28 installation:

I am using a python3 script to 1) get the token, and 2) login via ‘clientlogin’, but I always receive the   

"error": {

        "code": "badtoken",

        "info": "Invalid token",

The taken is coming from     'action': 'query',     'meta': 'tokens',     'type': 'login', and I strip the last ‘\’.

This is my submission for Login:

Type: <class 'urllib.request.Request'>

Contents: {'_full_url': 'https://wiki.xxxxat.com/api.php', 'unredirected_hdrs': {}, '_tunnel_host': None, 'fragment': None, '_data': b'password=Lxxxxxxxx7&action=clientlogin&rememberMe=1&username=Admin&returnurl=https%3A%2F%2Fwiki.xxxxat.com%2F&logintoken=8f2d58f9cc8336543c7f67cf8a5926d058a95ede%2B%5C&requestid=byMichael', 'origin_req_host': 'wiki.xxxxat.com', 'selector': '/api.php', 'headers': {'Content-type': 'application/x-www-form-urlencoded'}, 'type': 'https', 'unverifiable': False, 'host': 'wiki.xxxxat.com'}

Following  the same procedure via ApiSandbox is working fine.

Thank you for your help!