As alternative you can download the ssl certificate by hand. Afterwards pidgin starts without problems.
To download the certificate you can use openssl command line utility.
~/.purple/certificates/x509/tls_peers$ openssl s_client -connect YOUR_SERVER:PORTNUMBER
When the above command fails with "no peer certificate available" then maybe the server uses STARTTLS instead of SSL. In this case use the following command:
~/.purple/certificates/x509/tls_peers$ openssl s_client -connect YOUR_SERVER:PORTNUMBER -starttls xmpp
Now copy the part beginning with "----BEGIN CERTIFICATE----". If you print the content of the certificate file it looks like the following:
~/.purple/certificates/x509/tls_peers$ cat jabber.ulm.ccc.de
-----BEGIN CERTIFICATE-----
MIIFXDCCA0QCCQCa5jxvwccm0DANBgkqhkiG9w0BAQUFADBwMQswCQYDVQQGEwJE
RTEMMAoGA1UEBxMDVWxtMRUwEwYDVQQLEwxDQ0MgRXJmYSBVbG0xGjAYBgNVBAMT
EWphYmJlci51bG0uY2NjLmRlMSAwHgYJKoZIhvcNAQkBFhFqYWJiZXJAdWxtLmNj
...
3EIpMVk3V1djyj0FEuDaG/o+6BTLCiIMiIUFtbpVz8YZChHbv8ObMJ5JpUIkDfKZ
si1YZKpUYwpVXgTCUml67lArx/sq95OQsDSO3fR1Ch0=
-----END CERTIFICATE-----