1. “ERR_INVALID_RESPONSE”
If you see this error when you’re trying to download a file, please go to dashboard > Settings > General and make sure that WordPress Address (URL) and Site Address (URL) are both with https protocol.
2. Forbidden message on downloads
To fix this error please go to Downloads > Settings > Advanced > Miscellaneous and clear transients.
3. 404 Not Found
You might see this error because you do not have a file to download. This can happen if you imported downloads from a CSV file and the file’s path was not imported. To check go to your WP dashboard > Downloads > All Downloads > Edit your download > scroll down to Downloadable Files/Versions > Add File > Upload File > select your file > Update.
Another solution to try is to go to your dashboard > Settings > Permalinks > and save without changing anything.
4. Downloads do not work in Chrome
Go to your WP dashboard > Settings > General > make sure that at the WordPress Address (URL) and at the Site Address (URL) sections you have both the links written with https://