I'm using Mantis 2.11.1 on a Windows Server with IIS 8.5 and php Version 7.0.21. I have enabled the REST API ($g_webservice_rest_enabled = ON;), created an api-token and tested the API using Postman.
For the call http://srvmantis.polymania.com/mantisbt/api/rest/Issues with the header Authorization:myAPIKey I get '404 - File or directory not found' as result.