Short update: I further investigated the issue. The error which is returned from the baseband software is "access denied". It is in fact the same error code that is returned on the Motorola Moto G.
Funny enough, the executable that implements "native" Qualcomm rSAP is also existing on the Moto G. When I do exactly what is done in this executable, rSAP still fails, while it works fine on Sony, Xiaomi, ... This suggests that rSAP is in fact disabled inside the baseband software, maybe without a possibility to enable it at runtime. I was hoping that I only do something "wrong", which doesn't seem to be the case.
I hope this will not happen with all future Qualcomm baseband versions