Creating content based on importing html does not work!

Creating content based on importing html does not work!

Hello,
We are trying to import email templates to the platform for our regular campaigns, but with not success, simply trying to import a single HTML file gets stuck.
The same thing happens when we do a follow up campaign.

PD. Looking for an alternative way like coping and pasting the html code is not an option because it breaks the code and the responsiveness does not work.