To reproduce:

• Set up a job that checks out a project from git
• Set "Branches to build" specifier to something/somethingelse (that's valid in your git project)
• Try to build

This gives me the following error:
ERROR: Couldn't find any revision to build. Verify the repository and branch configuration for this job.

Doing the following makes the job build fine:

• git checkout -b something-somethingelse something/something else
• git push origin something-somethingelse
• Set "Branches to build" specifier to something-somethingelse