In case anyone experiences the same issue, I was able to resolve the problem. The cause was the url parameter I was using in the seraph-config.xml file. The article below discusses that file and seems to imply that you should use 'target' as the url parameter when using SSO. I tried target, redirectTo, and RelayState. But the solution was to use os_destination as the non-SSO line shows.
https://docs.atlassian.com/atlassian-seraph/2.6.1-m1/configuration.html