Somewhere between Jenkins 1.590 and 1.602, something broke wrt. the NextBuildNumber plugin. I've been replicating a number of freestyle multi-branch jobs as numerous freestyle simple jobs (b/c the multi-branch plugin is also no longer functioning properly), and needed to change the next build number to correspond to the numbers in the old multi-branch jobs. When you confirm the number change, the plugin never returns to the user, although it does alter the next build number. With default logging, nothing is logged - it just does not come back until you eventually get a browser timeout.