Answer
This is the PSO (Planning and Scheduling Optimization) integration's user-sync task — if you're not actually using PSO integration, the fix is simply to disable the corresponding Scheduled System Task (something like 'PSO Integration [USERS]') rather than trying to configure a working URL for it. Once that scheduled task is disabled and no longer running, the routing rules and other PSO-related configuration it would otherwise depend on don't matter at all.