Script Editor doesn't work: File not found (File test/test.groovy doesn't exist on the server filesystem)

Script Editor doesn't work after 5.6.13 upgrade.

Steps to reproduce:

• Go to <base url>/plugins/servlet/scriptrunner/admin/scriptEditor
• In Script Root tree click on any file

Expected behaviour

• File is opened in script editor pane

Actual behavior

• 'File not found' pop-up message appears: 'File test/test.groovy doesn't exist on the server filesystem'
• After clicking 'OK' another error appears:
  'File / doesn't exist on the server filesystem'