Did you import certificate as per the Adapter guide, then this may not work as it generates certificate for proxy.salesforce.com while enterprise WSDL will either have test.salesforce.com or login.salesforce.com. You need to download certificates for test.salesforce.com or login.salesforce.com via browser and then import those at runtime. Make sure your entire certificate chain is imported and valid.
There could be multiple reason of this error. Actual cause can be found at server detail logging. Reason could be -
1. credentials are not correct
2. certificate (*.Salesforce.com) has to imported into weblogic server
3. if you are not ale to access Salesforce.com from domain you are in (HOST_NOT_FOUND)
Please provide more details on this, I certainly should be able to help you.