Failed to load HTML file: status code is 400 (no replies)

Hello,

I´ve found an error on my piwik:

This is the error.... Failed to load HTML file: Please check your server configuration. You may want to whitelist "*.html" files from the "plugins" directory. The HTTP status code is 400 for URL "plugins/SitesManager/templates/index.html"

I would appreciate your help because I have not idea how to solve this issue

Best Regards