Googling on that in combination with AX2012 R2 didn't give any meaningful results. Even the mega AX MR solutions page didn't have an answer for this one...
I checked all permissions (and permission documentation), everything was 100% same as a working acceptance environment. No details in any logging... Permissions should be the same as I only updated the .LDF & .MDF locations using:
ALTER DATABASE ManagementReporterDM_PROD MODIFY FILE ( NAME = '<filename>', FILENAME = '<newdrive&path&filename>.mdf' );
While de DB was offline off course (ALTER DATABASE ManagementReporterDM_PROD SET OFFLINE;. )
I did not change any permissions here..!
Until... the google query "management reporter 2012 detach" brought me to this interesting article:
After removing (detaching & renaming the MDF & LDF) I asked the AX Admin to reinstall the RM DB from AX and... Everything worked again.
Hope the exact error and tags in this blog can help others that might face this issue in the future!