Thakns for getting in touch. Looks like you’re receiving a 500 Internal Server error from a Team City feed.
At this point, the only suggested course of action I have is to grab the teamcity-server.log logfile and try to find relevant log entries around the time you observed the error in Octopus. This ought to give some more insight into possible causes, perhaps a more descriptive error message or a stack trace.
One other possibility that comes to mind, I’m not sure what version of Team City you’re running, but the release notes here note that feed URLs did change as of 2018.1 so it may be worth looking at the new URL format and whether that responds correctly.