I’m not sure if this is the same issue, but we were receiving a similar issue after our upgrade when the path to the custom report went down another folder past CustomReports.
ex. before the path was “reports/CustomReports/foo/bar”, but when simplified to “reports/CustomReports/bar” it worked fine. I think it looks for the .rdl immediately after CustomReports.
I also had to reupload a few reports that were made in classic report style maintenance from Kinetic using Kinetic’s .zip file upload.