Some previous posts discuss about the error you get when you use the Admin Client in Jackson2 environments.

The current solution seems to be 

1. Revert to Jackson 1 by specifying it in the jboss deployment xml 

2. Use the Admin REST API directly.

Option 1 is possible if your application does not depend on any jackson2 feature (which is not the case for me and I suspect many others as well)

So we end using using the Admin REST API directly which is a shame as the AdminClient looks a more elegant option that we can use out of the box.

There also seems to be a Jira issue present for this

https://issues.jboss.org/browse/KEYCLOAK-815 

Any idea if this will get resolved anytime in the near future.

Cheers

Travis