Let's assume that multibranch_job is Multibranch job with defaults and normal_job is normal jenkins job.

If I set field in normal_job to:

Copy artifacts from another project -> Project name: multibranch_job/develop

all copying is working fine. The problem is when git branch is located in folder.

Copy artifacts from another project -> Project name: multibranch_job/feature/nameOfTheFeatureBranch

I still got errors: 
ERROR: Unable to find project for artifact copy:
This may be due to incorrect project name or permission settings; see help for project name in job configuration.