Got exactly the same error with a new ca-valid cert.
Problem was the format of the private key pem file :
In the PEM file, i got :
-----BEGIN PRIVATE KEY-----
instead of :
-----BEGIN RSA PRIVATE KEY-----
(diff from my old cert which was perfectly handled by openfire)
So, i had to convert my key file with OpenSSL :
openssl rsa -in privatekey.key -check
and paste the result to openfire.
Cert and intermediate certs go to the next field, in pem format, without any conversion.
That's worked for me.