I figured out, that the autopilot will not follow the LNAV-Profile sometimes. The active waypoint in the FMS wont update in that case aswell.
In VNAV-path, the descend-rates are pretty high at the beginning and the autopilot is ignoring overspeed, while trying to get the descend path (eventually a result, as the AP switches from vnav-idle to MCP-spd after a few seconds in descend).

Greets