A neat Let's Encrypt gotcha
This is a stupid problem, but it IS a problem. Recently we were trying to use a Let's Encrypt certificate on a third party webhost for one of our recent M&A ventures. We were constantly getting an error "Invalid Private Key Data", which was a head scratcher indeed.
Turns out, back in March of 2022 LE switched to using ECDSA as default (GO TEAM!!). This host had imply not updated their validation routine (Ummmmm...sweety...it's July 2023...). Switching the cert to RSA fixed the issue (thankfully this is just dev so no one cares it's RSA.)
So, if you too run into a lazy web host and get this error, try creating your certificate with the option "--key-type rsa"