thanks a lot for the reply.
I already contacted Laminar Research directly, but thought that maybe other people ran into the same problems. In the meanwhile I already found an alternative solution to the problem: By simply adding a 8-byte buffer of zeroes to the end of the command, X-Plane stopped complaining and everyone seemed fine. After further inspection, the whole command seemed to be shifted though. In the end i ended up changing the listed protocol to the following:
xint index + xint Buffer + xdob lat_lon_ele + xflt psi_the_phi + xint on_ground + xflt smoke_size + xint Buffer
4 Bytes + 4 Bytes + 24 Bytes + 12 Bytes + 4 Bytes + 4 Bytes + 4 Bytes = 56 Bytes
While does not represent the official command protocol, it still works out for my communication between Matlab/Simulink and X-Plane 11.