Even thou Subhas explained the current workaround, I think this bug should stay open until it's properly fixed:
The syntax for specifying job names when using manual trigger is the opposite of what one would expect: you must type the full job name, including the folder name, but without a leading slash. Normally, paths that don't begin with a slash are relative paths.
eg.: SomeFolder/UpstreamJob needs to specify SomeFolder/DownstreamJob as the downstream job name. DownstreamJob won't work (despite both being in the same folder) and /SomeFolder/DownstreamJob won't work either, even thou they all show up as Downstream Projects when viewing the job's page in Jenkins.
On the other hand, when using Parameterised Trigger Plugin, you don't need to specify the folder name. Just DownstreamJob works, and /FolderName/DownstreamJob also works. If Parameterised Trigger doesn't need a folder name to find the job, why does manual trigger need?