This is caused by DE53188.
Release : 188.8.131.52
Component : CA PPM INSTALLATIONS & INTEGRATIONS
This is logged as a Defect DE53188 and is under review by our sustaining engineering team. You can use the below workaround.
1. On the PPM server, navigate to $NIKU_HOME/tomcat-app-deploy/webapps and copy pm.war file.
2. Paste the pm.war file copied in step 1 to $NIKU_HOME/tomcat-app2-deploy/webapps.
3. The file gets autodeployed as soon as it's copied and if you access New UX on the app2 service it works.
Note :- If you remove the service and re-add it, you have to perform the workaround again.