Please see attached Screenshot.
What does that mean and how to solve it?

AC: Phantom 4 Pro, NOT actual Firmware due to incompatibilty with Autoflight Logic and Drone Deploy. (im on the previous Firmware)
Setting on GSP was for 3D-Map, Flight Length: 1600 Meter, Waypoints Qty: 267 PTS.
Could it be, that GSP is limited to 99 Waypoints?

Yes, that is exactly it.

Also, the only work around is to decrease your area, overlap, or edit the mission to shoot on a distance or time interval. Each place where the UAV must stop and hover in order to collect an image is a waypoint.