What I do get is the following error message when I tried to reinstall the addon manager:
Excluding the files in the FSX folder is not enough because, BEFORE a file is installed in the FSX folder, it's first uncompressed in the temporary folder so, that message seems to indicate the antivirus is interfering in THAT folder.
That's why, we always suggest to TURN OFF the antivirus ENTIRELY, when DOWNLOADING and INSTALLING. Excluding the whole %TEMP% folder is a bit dangerous to do so, it's best to turn off the antivirus while installing.
And, you should always restart Windows before trying to reinstall after getting an "access denied" error.
Question is: How do I fix the issue permanently?
Switch to a non-bugged antivirus that is not mistakenly detect our software as threats. That would fix it permanently. The free and always updated Microsoft Security Essentials is one example of a working antivirus that doesn't require *any* configuration to install our products.