Resolution:
The client had SARINIT parameter "TAPEOPT=3480,...", which means that archival tapes are written using 3480 (includes 3490 and 3590) data block location processing.
When doing retrieval of a report from tape with that setting, you need first to do a SARBCH /LOAD of the report to the disk layer.
Client changed the parameter to "TAPEOPT=EAST,..." and (as they are licensed for EAS) were able to access and browse the report as expected.