I experienced this problem for the last 2 or 3 versions of Domino Leap but today I found out the reason.
I always use Domino container images built using the HCL community script and I always add Nomad Web and Leap. The latter suddenly stopped working I think 2 or 3 versions ago. Until now I was not able to track it down since no useful logs were written. With version 22.214.171.124 I at least see some logs in the error-log-0.xml file.
If you happen to have this problem, too, just check the log file mentioned above for something like this:
com.ibm.form.platform.service.framework.exception.ServicesPlatformException: java.lang.NoClassDefFoundError: groovy/lang/Writable
I am using several OpenNTF plugins on my machines, and also SmartNSF was one of those enabled in my update sites NSF. As I am not really utilising this feature, I disabled this for the sake of testing - and et voilà, Leap works as expected.
I don’t even know why I came across this again and it bothered me (since I didn’t use Leap that much) but now I solved this issue.
I hope this helps you, too.
By the way, with the newest version of Leap you have to enable these roles for your app manager in the Volt Builder NSF to get the new Admin area: