I can connect to a 1.15 kadmind with the 1.17 kadmin.  While there may be a behavior difference leading to a connection error in your deployment, there is not a universal incompatibility.

Please collect trace logs of the 1.15 and 1.17 kadmin clients, by setting the KRB5_TRACE environment variable to a filename ("KRB5_TRACE=/path/to/filename kadmin").