If you supply a value greater than 1 or smaller than -1 to the acos function, you get a complex number. Simulink seems to be complaining about that in your code.
Seems like at the very first time step, argumancos5 is exactly 1, but due to numerical issues, it's actually slightly higher than 1 which leads to your error. Computing the value argumancos5 - 1 gives me 3.1086e-15... so it's just outside the range in which the cosine is real-valued.
I limited the value of this input to the range [-1, 1] and it seems to have fixed the error:
argumancos5 = max(min(argumancos5,1),-1);