Also having this issue with a PositiveSSL/ComodoSSL cert via Namecheap.
Tried the command @Stynson suggests:
openssl verify -CAfile mysite.ca-bundle /path/to/your/mysite.crt
Results in OK
One difference is that my key type is PKCS7 (.p7b file)
My guess is that it the certificates are ok, but that Netlify is having trouble parsing the key format. Perhaps that is leading to some decoding issues during key checking, I’m not sure.
EDIT: For me the issue was quite silly. I was using the pkcs7 file as the key, which I think is actually some kind of alternate encoding for the certificate file(s). I’m not sure if this will be helpful but I will keep it here in case someone else is confused about all the different file types.