How do i login a webservice user from java?

I build a custom request handler that handled incoming requests with BasicAuthentication. Because i did not want to bypass the Mendix security i added a piece of code that tries to login and generates a user context based on that. However, I get an error that a webservice user is not allowed to login.

I use the following snippet to achieve the above:

final String[] values = credentials.split(":",2);    
IContext context = Core.login(values[0],values[1]).createContext();