Error loading stylesheet

I have been using this extension for quite some time successfully. However, recently when I try to call the sitemap with Firefox, I get the following error:

Error loading stylesheet: An unknown error has occurred (805303f4)
https://static.cucocreative.co.uk/plugins/google-sitemap-generator/sitemap.xsl

We serve all our our static content via a cookieless domain, which I think may be the root cause. In Chrome console you get the error:

Unsafe attempt to load URL https://static.cucocreative.co.uk/plugins/google-sitemap-generator/sitemap.xsl from frame with URL https://www.cucocreative.co.uk/sitemap.xml. Domains, protocols and ports must match.

But I don’t know how to fix this issue? I would greatly appreciate any help.

I have found these theads but they offer no solutions:
https://wordpress.org/support/topic/805303f4/
https://wordpress.org/support/topic/error-loading-stylesheet-an-unknown-error-has-occurred-805303f4-7/