I have problems solving a geometric program

To begin with, you need a solver that can solve geometric programs (YALMIP currently supports the native but old GP-solver GPPOSY, MOSEK which is called via an exponential cone formulation, and FMINCON, KNITRO, IPOPT, and SNOPT by internally performing a variable change to write the problem as a convex problem over logarithmic variables).

If you still have problems, the reason can be that YALMIP converts convex quadratic constraints to second-order constraints. This should not be done in geometric programs (it is a known bug). To avoid this, set the option ‘convertconvexquad’ to 0. Another reason is that runs into problem during the expansion of nonlinear operators. To avoid this issue, explicitly tell YALMIP that the problem is a geometric problem by specifying the solver to ‘gpposy’, ‘mosek-geometric’, ‘fmincon-geometric’, ‘snopt-geometric’ or ‘ipopt-geometric’.

You an alternatively force YALMIP to avoid any convex reformulation (it works surprisingly well) and use standard nonlinear solvers ‘fmincon-standard’ etc on the original signomial form.