Hello José,
do you have access to the server logs? The server should have all the details about the error.
From what you post here I assume that it is a problem of misinterpretation of the encryption algorithm. For example, you take a PKCS#8 certificate, but interprete it as PKCS#1. I found this regarding the leading zeros:
So it looks like your server expects a PKCS#1 with a leading zero like in #2 in that article, but you send a different format that has no leading zeros.
Your soapUI might have different settings in this regard. So the certificate is certainly correct, but the settings must have an error somewhere.
Hope that helps,
Jörg