Try starting a fresh MATLAB session. Perhaps there is a conflict due to having previously used the original CVX version of exponential.m in the same session in which you later try to use the CVXQUAD version.
Please be sure you are using CVX 2.1, not CVX3.0beta. Deinstall 3.0beta and install CVX2.1 if necessary.
If that doesn’t solve your difficulty, you can try opening an issue at https://github.com/hfawzi/cvxquad/issues where the CVXQUAD developer can help you.