You see 400 bad request for Password Service Task visible in network trace for the password change request.
identity Manager v15.
This is a known issue in Siteminder.
Please upgrade Siteminder as per information in the article.
Workaround for the issue is to set ALLOW_UNESCAPED_CHARACTERS_IN_URL to true,
Add below JVM options to your IDM. Under your IGA Xpress --> Services --> Identity Manager --> Java Options
-Dorg.wildfly.undertow.ALLOW_UNESCAPED_CHARACTERS_IN_URL=true
IF this is not working change it directly by editing jboss.xml file adding entry
<http-listener name="default" socket-binding="http" allow-unescaped-characters-in-url="true" (...)>
<http-listener name="default" socket-binding="https" allow-unescaped-characters-in-url="true" (...)>