SSL certificate verification fails, how to figure out what's causing it?

Question

I've recently obtained a PositiveSSL certificate at Namecheap and installed it on my server. Accessing the site from Firefox works fine, but accessing it from Ruby's net/https library doesn't work: it fails to verify the connection certificate even though I've specified the path to the certificate and I've checked that the file is readable. Curl also fails:

curl --cacert /path/to/cert https://mysite.com/

It simply says something like this:

curl: (60) SSL certificate problem, verify that the CA cert is OK. Details:
error:14090086:SSL routines:SSL3_GET_SERVER_CERTIFICATE:certificate verify failed
More details here: http://curl.haxx.se/docs/sslcerts.html

curl performs SSL certificate verification by default, using a "bundle"
 of Certificate Authority (CA) public keys (CA certs). If the default
 bundle file isn't adequate, you can specify an alternate file
 using the --cacert option.
If this HTTPS server uses a certificate signed by a CA represented in
 the bundle, the certificate verification probably failed due to a
 problem with the certificate (it might be expired, or the name might
 not match the domain name in the URL).
If you'd like to turn off curl's verification of the certificate, use
 the -k (or --insecure) option.

"certificate verify failed" isn't a terribly useful error message. How do I find out what exactly is wrong with my certificate and what to do about it? I find it confusing that it works in the browser but not anywhere else.

Answer 1

It looks like curl requires that the CA certificate file contains ALL certificates in the chain. I've downloaded all of them and combined them into a single file and now both Curl and Ruby are happy.