Open Sans

[dlopez2000]

I tried https://nextgenthemes.com/google-webfont-downloader/ on:
https://fonts.googleapis.com/css2?family=Open+Sans:ital,wght@0,400;0,600;0,700;1,400;1,600;1,700&display=swap
and the resulting page was blank instead of the usual download page. Not sure what your internal error is.

btw- I came across your site because I tried downloading the fonts manually and was surprised to see Google serving up different woff2 files depending on whether I was using Chrome or Firefox... I thought they were standard! These are both Open Sans 600 latin:
Chrome: https://fonts.gstatic.com/s/opensans/v27/memvYaGs126MiZpBA-UvWbX2vVnXBbObj2OVTS-muw.woff2
Firefox: https://fonts.gstatic.com/s/opensans/v27/memSYaGs126MiZpBA-UvWbX2vVnXBbObj2OVZyOOSr4dVJWUgsgH1x4gaVI.woff2
Is this well-documented behavior by Google? Does your tool take this into account?

[nextgenthemes]

This is about font hinting. When I read up on this they served hinted and non-hinter versions of the fonts. I use a windows user agent to download the fonts with this script. I think this downloads the hinted versions that are slightly bigger but I do not remember.