Doing this shows there is an attribute named 'path':

subsystem=deployment-scanner,scanner=default:read-resource
{
"outcome" => "success",
"result" =>

{ "path" => "deployments", "scan-enabled" => true, "scan-interval" => 5000, "relative-to" => "jboss.server.base.dir" }

,
"compensating-operation" => undefined
}
but reading that attribute fails:
~] subsystem=deployment-scanner,scanner=default:read-attribute(name="path")
{
"outcome" => "failed",
"failure-description" => "No known attribute called path"
}

When using http://localhost:9990/domain-api/subsystem/deployment-scanner/scanner/default?operation=attribute&name=path
the same result is returned.

In error log it shows:
16:44:53,691 ERROR [org.jboss.as.controller] (pool-4-thread-2) operation ("read-attribute") failed - address: ([
("subsystem" => "deployment-scanner"),
("scanner" => "default")
]): java.lang.NullPointerException
at org.jboss.as.server.ServerControllerImpl.doExecute(ServerControllerImpl.java:180)
at org.jboss.as.controller.BasicModelController.execute(BasicModelController.java:221)
at org.jboss.as.controller.BasicModelController.execute(BasicModelController.java:187)
at org.jboss.as.controller.BasicModelController.execute(BasicModelController.java:171)
at org.jboss.as.controller.AbstractModelController.execute(AbstractModelController.java:56)
at org.jboss.as.domain.http.server.DomainHttpServer.handle(DomainHttpServer.java:95)
at com.sun.net.httpserver.Filter$Chain.doFilter(Filter.java:82) [:1.6.0_22]
at org.jboss.sun.net.httpserver.AuthFilter.doFilter(AuthFilter.java:80)
at com.sun.net.httpserver.Filter$Chain.doFilter(Filter.java:85) [:1.6.0_22]
at org.jboss.sun.net.httpserver.ServerImpl$Exchange$LinkHandler.handle(ServerImpl.java:606)
at com.sun.net.httpserver.Filter$Chain.doFilter(Filter.java:82) [:1.6.0_22]
at org.jboss.sun.net.httpserver.ServerImpl$Exchange.run(ServerImpl.java:578)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886) [:1.6.0_22]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908) [:1.6.0_22]
at java.lang.Thread.run(Thread.java:680) [:1.6.0_22]