Assuming you are not trying to install the Addon Manager while FSX is running ( I hope you don't... ), the only other possible reason for this error is the antivirus, which is mistakenly blocking the installation of the file.
- Reboot Windows
- Install the Addon Manager with the antivirus disabled. Be sure you don't run TWO antivirus at the same time, for example the default Windows Defender and a 3rd party one. If you do (don't, because it will kill performances), you must disable them all.
- Also, configure the antivirus to exclude the whole FSX folder from scanning.
Of course, the "real" solution, should be switching to a more reliable antivirus, one that doesn't mistakenly flag legit programs digitally signed with anti-false positive standard signatures (like our own) as threats.
