If you had a static inbound agent configured in 2.426.x, there is a special compatibility mode for the config screen which allows a GUI round-trip (Configure + Save) to preserve fields which are now deprecated such as webSocket. In the case of tunnel this failed to call fixEmptyAndTrim, so that if no tunnel was specified (blank text field) the config.xml of the agent would nonetheless contain <tunnel></tunnel>, causing the proposed launch instructions to include -tunnel (with no argument) which is incorrect.