"Repository/List Modules" is equivalent to command line "cvs checkout -c". Check the Cederqvist CVS manual, Appendix A.7.1. "cvs checkout -c" lists the contents of the modules file of a repository.

The modules file can be used to define modules as e.g. subsets or combinations of other modules. See the Cederqvist, Appendix C.1.

The fact that you don't see any output from "List Modules" suggests that you have not defined any such custom modules in your repository. The "real" modules you get when you e.g. import something are not automatically added to the modules file. Thus, even in a non-empty repository the output of "List Modules" can be empty.