disable pss in Networker 9.1 not working as expected

I would like to use checkpoints on a Networker 9.1.0.5 win_x64 client with Networker 9.1.0.5 lin_x64 server.

So I have checked "checkpoint enabled" and "checkpoint granularity = directory" in client settings. I also have "Parallel save streams per save set = disabled" and "Physical client parallelism = disabled" and "Parallelism = 1".

Nevertheless checkpoint feature won't be used. I guess somehow disabled pss settings are not honored.

Disabling checkpoint restart for client xxx: this feature is not supported for aggregated save.

Any idea how to convince Networker using checkpoints?

Regards, Otmar