As @Erling wrote, CVX 3.0 beta is very buggy. It does have the ability to use the native exponential cone capability of SCS and ECOS. But there is no build of CVX which currently uses Mosek 9.0’s native exponential cone capability.
I recommend you use CVX 2.1, with Mosek 9.0 as solver, and install CVXQUAD and follow the guidance at CVXQUAD: How to use CVXQUAD's Pade Approximant instead of CVX's unreliable Successive Approximation for GP mode, log, exp, entr, rel_entr, kl_div, log_det, det_rootn, exponential cone. CVXQUAD's Quantum (Matrix) Entropy & Matrix Log related functions . I see from a previous question that you have already installed CVXQUAD.