To edit plugin settings, your admin user must have the manage_wallets capability. Normally this is assigned to you automatically, but can you double check with a capabilities plugin? This is the most likely cause of this problem.
Also, make sure that you have refreshed the settings page before hitting “Save changes”. If the page is cached in your browser it might contain a stale wordpress nonce.
Finally, it could be some security plugin you are using that interferes. Try disabling it for a moment to check if it makes a difference.