SQL & PL/SQL

Hi all - can anyone shed light on this please.

We're trying to implement the following call using UTL_HTTP to a https-based site (in fact web service):

select utl_http.request ('https://###Secure URL###', NULL,
'###WALLET LOC###', '###WALLET PASSWORD###') from dual;

We followed the recommendations of Metalink note 169768.1 and obtained the necessary certificate from the site and loaded it into our Wallet Manager. We have had success with a couple of test https URLs and their associated certificates, such as:

https://metalink.oracle.com
https://scgi.ebay.com/ws/eBayISAPI.dll?RegisterEnterInfo

However when we try the actual Web Service endpoint URL that we need, we get the following error:

ERROR at line 1:
ORA-29273: HTTP request failed
ORA-06512: at "SYS.UTL_HTTP", line 1577
ORA-28857: Unknown SSL error
ORA-06512: at line 1

The ORA-28857 error has no information to support its resolution - its one of the 'you shouldn't get this'-type messages. Can anyone shed any light on this? The URL we're trying to reach is: https://hts.maynegroup.com/pharmacy_ordering_dev/PO.asmx

Any help or suggestions are welcome. Our only thought is that the certificate is somehow invalid...

Kind regards
Dennis