Utl_http does not give a very helpful error message: Invalid Parameter

Question

I'm writing PL/SQL to consume a web service on another server. In trying to execute this snippet of code, I receive this error:
ORA-28783: Invalid parameter
ORA-06512: at "SYS.UTL_HTTP", line 1023
ORA-06512: at line 29
Line 29 is the utl_http.begin_request statement shown below.
I'm baffled as to what the error message might point to and I'm looking from any insight others may have. The message is so generic as to be useless and I can't find any examples of others receiving this particular error when executing begin_request.
Please note that this is an SSL/HTTPS connection. If I remove the "S" and just use HTTP, I receive an appropriate message from the web server. (A tiny web page telling me I should be using the https version and not the http version.) I can also go to, for example, Google or CNN's web sites and pull back data but those are not SSL connections. It is the SSL part that isn't working.
This isn't a SOAP error since I never get a chance to create my SOAP message and place it in the request.
I've tried changing the POST to GET and the HTTP version to 1.0 just to see if the error message changes and it does not.
My wallet file is in place on the database server, placed in an accessible folder by
the DBAs. This code can see the wallet, I believe, because if I change the wallet
path to something non-existent, I receive a different error. I imported the web site certificate for the site I'm connecting to and placed that in the wallet. I've been able to cause "certificate chain" errors with empty wallets, so I think the certificate is correct.
I used SOAP-UI (www.soapui.org) and was quickly able to connect to the web server and begin having a "SOAP conversation", so I know the web services I'm connecting to are working. It is something in the database itself that is preventing this from executing - the wallet, a setting in the database, something like that. But what?
=======================================================
utl_http.set_response_error_check(enable=>true);
utl_http.set_detailed_excp_support(enable => TRUE);
utl_http.set_follow_redirect(5);
utl_http.set_wallet('file:/...', '*******');
--**** SET THE URL FOR THIS REQUEST *******
http_req:= utl_http.begin_request(
'https://....' --location of web services I want to use
, 'POST'
, utl_http.HTTP_VERSION_1_1
=======================================================

Answer 1

You seemrd to have ruled out most potential problems I could think of.
The error message just states that there is something wrong with the parameters that you use. Only point I can currently think of is a problem with the returned value.
How did you declare you http_req variable? Does it use the utl_http.req type?