Similar to what is seen here at this communities post,
The login to OTK Manager will fail and display only html for multiple reasons including JDBC connection problems.
If the login page is failing to load anything BUT raw HTML and ssgrestore.sh was used to restore this system please review the resoloution data.
If ssg restore has been run it may modify some file permissions.
The first fault may cause the gateway to not start (wont run or log any data).
Which can be fixed by,
Ensuring/adding additional permissions to files under,
Once the ssg service is running you may get this manager html return.
This can be caused by missing permissions on the OpenIDConnectAssertion
(which causes the DecodeID token to show as custom code not available).
And it can be corrected by,
1) Login to the gateway.
2) chmod 444 /opt/SecureSpan/Gateway/runtime/modules/lib/OpenIDConnectAssertion-b753.jar
3) service ssg restart
4) After startup is done hit the oauth manager