I've updated and installed my GSX to version 2.2.021 and installed a new Addon Manager 4.0.0.6
First, the current version of the Stand-Alone Addon Manager is 4.0.0.8.
And, as explained several times on the forum in other threads, installing the Stand-Alone Addon Manager is not entirely useless, since the FSDT Live Updater, if run when the sim is closed, can
Update everything from FSDT, without requiring to do anything else.
Not that installing the outdated Stand-Alone Addon Manager could cause any issue, since the 4.0.0.6 is still recent enough to do the "Live Update" step at the end so, it won't cause problems, but it's just a wasted effort.
About your issue, try to Reset the configuration for the airplane because, we have updated the configuration that comes with GSX for the Captain Sim 757 to have a separate Main Cargo door, but if you customized the airplane in the past, your custom configuration is taking precedence, preventing GSX from using its internal one.