It's not possible this has been caused by the 1.3 update, because the Addon Manager .DLL hasn't change AT ALL, not a single bit, and it has been a while since it hasn't been updated.
So, either you had an old version and never run any installer recently (at least since a couple of months) because that would have got you exactly the same version OR something else, like the antivirus, has corrupted the file on your system, so it doesn't work.
Try to allow the module to run, even if FSX crashes, try a few times, then reboot Windows again, and if still doesn't work, try uninstalling/reinstalling, and reply YES to the question "Do you want to remove the Addon Manager ?" when uninstalling, then reinstall again.