2476639 - "Cannot authenticate the user" exception in REST Adapter

After updating PI 7.31 to Support Package 17, the REST adapter communication is failing with "Cannot authenticate the user" or asking for credentials, even though the channel is configured to "Allow public access (no authorization required)".

By taking an XPI Inspector (1514898 - "XPI Inspector for troubleshooting XI") with example 50, it is possible to see the following errors:

Error during login
[EXCEPTION]
com.sap.engine.services.security.exceptions.BaseLoginException: Cannot authenticate the user.
at com.sap.engine.services.security.login.ModulesProcessAction.run(ModulesProcessAction.java:134)
at java.security.AccessController.doPrivileged(Native Method)
at com.sap.engine.services.security.login.FastLoginContext.login(FastLoginContext.java:285)
at com.sap.aii.adapter.rest.ejb.auth.CustomAuthenticator.authenticate(CustomAuthenticator.java:75)
at com.sap.aii.adapter.rest.ejb.RESTAdapter.service(RESTAdapter.java:110)
at com.sap.aii.adapter.rest.web.RESTServlet.service(RESTServlet.java:56)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:847)
at com.sap.engine.services.servlets_jsp.server.Invokable.invoke(Invokable.java:152)
at com.sap.engine.services.servlets_jsp.server.Invokable.invoke(Invokable.java:38)
at com.sap.engine.services.servlets_jsp.server.HttpHandlerImpl.runServlet(HttpHandlerImpl.java:466)
at com.sap.engine.services.servlets_jsp.server.HttpHandlerImpl.handleRequest(HttpHandlerImpl.java:210)
at com.sap.engine.services.httpserver.server.RequestAnalizer.startServlet(RequestAnalizer.java:441)
at com.sap.engine.services.httpserver.server.RequestAnalizer.startServlet(RequestAnalizer.java:430)
at com.sap.engine.services.servlets_jsp.filters.DSRWebContainerFilter.process(DSRWebContainerFilter.java:38)
at com.sap.engine.services.httpserver.chain.AbstractChain.process(AbstractChain.java:78)
at com.sap.engine.services.servlets_jsp.filters.ServletSelector.process(ServletSelector.java:81)
at com.sap.engine.services.httpserver.chain.AbstractChain.process(AbstractChain.java:78)
at com.sap.engine.services.servlets_jsp.filters.ApplicationSelector.process(ApplicationSelector.java:278)
at com.sap.engine.services.httpserver.chain.AbstractChain.process(AbstractChain.java:78)
at com.sap.engine.services.httpserver.filters.WebContainerInvoker.process(WebContainerInvoker.java:81)
at com.sap.engine.services.httpserver.chain.HostFilter.process(HostFilter.java:9)
at com.sap.engine.services.httpserver.chain.AbstractChain.process(AbstractChain.java:78)
... 45 more

• SAP NetWeaver
• SAP Process Integration

Process Integration 7.31, SP17, PI 7.31, support package 17, cannot authenticate user, authentication issue, public access, authentication failed, XI 7.31, asking for authorization, Unable to post to the public, public access without authentication, asking credentials , KBA , BC-XI-CON-RST , Rest Adapter , BC-XI-CON , Connectivity , Problem