I'm getting tons of issues on my https site after exporting from Webflow because, for example, you would use scripts from external resources that are only http rather than https (when they SUPPORT https).
Just CTRL+F through the index to compare. For example,
should add a 's' to the end to prevent SSL issues when exporting - they support it, so why not
Be weary of others.
EDIT: hmm webflow's io sites are https as well ... I never checked the log. Perhaps this is even being blocked for your sites!