OK, got it working now. Thanks for the info, scoobflight. I think my problem was I have not introduced the cruise altitude to the FMC during the performance initialization.
Just a little background for your interest:
I actually bought the 135/145 in Sep 2011 as back that time I was flying a lot with Luxair who operated these aircraft on regional routes across Europe. As a passenger, I enjoyed the aircraft a lot. It felt like a private business jet :-)
However, I installed it and started to do some flying in P3D only now (yes, some 7 years later!). In the meanwhile, Luxair sold the 135/145 fleet and replaced it with Dash Q400. The passenger experience is not as good but I suppose, the swap made sense in economical terms.
Anyhow… now I enjoy the 135/145 again. I like learning the systems and the workflow. Thanks for your work and your support!
Milan