Issue was:
Tasks were created with little relevance to support filtering the target system
Even though the task was saved in a custom site with only windows systems subscribed, when action is targeted to all systems it does not apply the custom site subscription filter until after the initial task relevance is evaluated, which is why it yielded the unable to access the site.
As usual this was a previous well known issue but not to me:
The same topic was also discussed in the following forum link: Fixlet running against PC's not in site
The resolution was to add the platform relevance and other relevance to the task itself and not expect the site subscription to perform the filtering.