Hi @gmontiel,
I’ve had this issue a few times with both the Classic Editor and Gutenberg. Could you please try copying the embed code from the site you’re trying to embed from? Then paste that embed code into an HTML block. A site’s embed code usually works better for me than letting WordPress figure it out.
If this trick doesn’t work, then there may be a plugin or theme conflict that’s preventing the embed from displaying correctly. In that case, please attempt to disable all plugins, and use one of the default (Twenty*) themes. If the problem goes away, enable them one by one to identify the source of your troubles.
If you can install plugins, install and activate Health Check: https://www.ads-software.com/plugins/health-check/
It will add a new feature under Tools > Site Health.
On its Troubleshooting tab, you can Enable Troubleshooting Mode. This will disable all plugins, switch to a standard WordPress theme (if available), allow you to turn your plugins on and off, and switch between themes, without affecting normal visitors to your site. This allows you to test for various compatibility issues.
There’s a more detailed description about how to use the Health Check plugin and its Troubleshooting Mode at https://make.www.ads-software.com/support/handbook/appendix/troubleshooting-using-the-health-check/.