An '$i$' is missing in Eq. (10c), so these coefficient should read:
$$L_{\pm}+iK_\pm=\frac{\frac{8\Omega_\mathrm{L}^2}{\gamma_\sigma(\gamma_\sigma+\gamma_\phi)}\big[1 \pm i \frac{5\gamma_\sigma-\gamma_\phi}{4 R_\mathrm{L}}\big]-\frac{\gamma_\sigma-\gamma_\phi}{\gamma_\sigma+\gamma_\phi}\big[1\pm i\frac{\gamma_\sigma-\gamma_\phi}{4R_\mathrm{L}}\big]}{4\big(1+\frac{8 \Omega_\mathrm{L}^2}{\gamma_\sigma(\gamma_\sigma+\gamma_\phi)}\big)}$$