Roberto,
I also had this error once, but I don't remember how I fixed it. The hint though is that when trying to mount acs-lang it is trying to get the pretty_name of that package from apm_enabled_package_versions. However, there is no enabled package version acs-lang which means the install of acs-lang previously failed for some reason.
Do you have any error messages prior to the one you posted? Also, make sure to do another cvs update as we committed a whole bunch of stuff yesterday and will continue to commit today. Our dev server should be pretty much in sync with cvs head and we reinstall it quite often on both Oracle and PG.
