Not much advice we can give on the limited information. Would recommend getting set up to collect the logs the Jabber plugin writes to the file system, described here: https://developer.cisco.com/site/jabber-websdk/develop-and-test/voice-and-video/troubleshooting/
If you cannot spot the issue (the logs are pretty detailed), you will likely want to open a ticket with DevNet Developer Support for in-depth troubleshooting help: https://developer.cisco.com/site/devnet/support
Please be prepared with details of the call scenario when the error occured, including phones/DNs involved, timestamps, call sequence, etc. Also note CUCM and Jabber plugin/SDK versions.
