Thanks for this info, but we are on SAPKW74012 (and the note refers to sub-11).
Activating authorization checks ST01 and RSECADMIN created no fail records in their respective logs (currently nothing has been marked as authorization relevant anyway). I assume that this is what you refer to with "checked the authorizations from a BW perspective".
Given that the object is a HANA object (a composite provider created in HANA Studio) and the error pops up when trying to execute an SQL statement in HANA Studio on the composite provider I assume this is not surprising.
The mystery remains!