I just came across the warning referring to this ticket, on a Jenkins deployment with no executors on the Jenkins node (which should not happen).
JENKINS-2111 path sanitization ineffective when using legacy Workspace Root Directory
There are actually two possible sources of this warning in the Branch API Plugin itself, checked on v2.6.2.
If the message is immediately followed (assuming you have FINE logging visible, otherwise you won't see it) by
> no available workspace root for hudson.model.Hudson@...
then it's come from a call to locate, which will happen when a job is deleted or renamed, as the Deleter runs a task on each node (jenkins.getNodes()), which will trigger a call to locate for the Jenkins node, which hits this warning, returns null, and is ignored.
If no such message followed the warning (and you have FINE logging visible), then it might have come from Collector reacting to the Jenkins node coming online and calling getWorkspaceRoot directly, which will again return null and be ignored.
This warning means it won't clean up orphaned/renamed workspaces on the Jenkins node, but if you have no executor there, then you shouldn't have any to clean up anyway.
Edit: I just noticed that JENKINS-60451 was logged for exactly this.