As it turns out, there are two binary files in the ZIP file:
flash_aod_RK055AHD091.bin
flash_aod_RK055MHD091.bin
*NOT* according to the description of the AN, you have to flash the second one, with the "M" in it. Works now.
Is there any chance to get the sourcecode of the M33 binaries? I cannot find it in the examples of the SDK.