I can only repeat and confirm, the one and only possible reason for that error is the antivirus. I can also assure you that, if you really use *only* Windows Defender, excluding the Addon Manager folder is the only thing required, nothing else.
Be sure you don't have another security program that might do similar things, products like firewall, internet security, anti-spyware and such might do a realtime protection ( and thus catch false positives too ), like an antivirus, even if they are not technically antivirus, so you must have to configure them as well.
