Usually this error message is seen if you have written a complicated nonlinear expression that is defined over a very large domain. For example, a sum over large domains where the expression inside the sum goes on for pages. A definition of an equation over a large domain can have the same effect. Everything for that block of equations has to generate intermediate code in the same address space.
There are two possibilities. One is that you are generating something much larger than expected or necessary. The other is that your model is correct but very, very large.
The generic advice for a work-around is to introduce intermediate variables to get a smaller code size per block of equations. Another thing to consider would be to partition the domains so that you might, for example, have two blocks where each is half the size.