Hi Laurent,
I found a bug in the OpenSSL.pm. It only occurs if you have the private key encrypted.
The same certificate with manually decrypted private key is working without any problem.
The validation in OpenSSL.pm function validatePrivateKey is ok.
The problem only occurs at the function importPrivateKey, because the process is wating for the passphrase and the dialog window shows only this error and nothing in the logfiles.
When you check the process you could see, that the passphrase parameter is missing.
I checked the code in OpenSSL.pm but for me it is ok, so I don't know why it is not working.
You know your code better, maybe you will see the problem.