Thank you for your e-mail. This could have a multitude of root causes, the 403 is quite general. One cause is of course the AWS WAF either from customer specific settings or source reputation classification.
I would advice to contact the site admin to see what the cause is, we do not have access to check the application level. For the reputation classification I would advice to contact for instance Neustar to confirm the current status, and work from there.

However, I find it impossible to contact all the individual sites that are no longer functioning for us.
If anyone has a better solution, I would appreciate your input.

Thanks,
Quentin.