User unable to access drilldown report, but has access to the parent report. The user sees a "Report visible only to a specific user or group" message
The same drilldown report is associated to multiple parent reports. This is incorrect as a drilldown report should only belong to 1 report
Ensure the drilldown report is only associated to 1 parent report.
You will need separate drilldown reports for each report. It is not supported to have 1 drilldown report belong to multiple reports.
The issue lies with the reports who the have same drilldown report because when the ACL for 'sys_report_drill' gets evaluated and runs the line "rootReport.canRead()" the system gets confused about which it is evaluating as the "rootReport" to allow access to the user. A drilldown report must only be associated with 1 report, this will prevent the issue you are seeing.