Symptoms
When trying to access a custom report when logged in with a user role that only has 'normal' privileges for reports (list reports = allow / everything else = disallow), a '403: Unauthorized' message is displayed.
Cause
There is a bug in version 2015 R1, R2, and R2 SP1 that prevents users from accessing custom created reports when logged in with only read-only report permissions.
Resolution
This has been fixed in FlexNet Manager Suite 2015 R2 SP3.
Workaround
In addition to 'List reports', the permission to 'Modify an existing report' must also be granted in order to be able view custom reports.