Hmm, so after transporting the report via OMA to sandbox, it is exporting as XLS in sdb system? Both systems are at same patch level I presume.
If the issue is only in PROD, you can try one thing to exclude the report itself:
- get OMA exports for both SDB and PROD system for that report
- extract OMA (as zip) and compare the xml files inside with a DIFF tool
- check for any differences (excluding object ids); maybe some config is not transported correctly via OMA to sandbox
Bogdan