When a READONLY client is synced with the "Auto cleanup and sync" option, the reconcile throws an innocuous "Client of type 'readonly' cannot modify files." error. In 1.10, this error is ignored. In 1.10.3, this error causes the build to fail.
This may be a consequence of a modification to src/main/java/org/jenkinsci/plugins/p4/client/ClientHelper.java in Change 25759, which added an exception when a reconcile fails. Unfortunately, in the above scenario, reconcile shouldn't even be called, so the fact that it fails should be ignored.