First of all many thanks to Patrick Wolf for explaining me the reason of this bug and contributing a quick workaround.
The story goes as follows:
if you install Apex 4.2 on a fresh database or upgrade an installation of Apex 4.1 where there are no existing workspaces and you plan to import them later from saved files exported from earlier versions, your plan is going to fail miserably showing an error page like this:
The failure may happen either when creating users or groups, if there are no groups defined, it will fail when attempting to create users.
The reason lies in a new API parameter introduced in Apex 4.2 that is obviously missing in the old export files.
As you can imagine, by manually adding the missing parameter to the old dump files, does the trick.
Locate the following API call in the workspace dump files (parameter values of course will be different for you):
and make sure to add the line as shown in the picture (don't forget to add the comma after the last parameter.
p_allow_restful_services_yn => 'Y'
Oracle also made available patch 14777994 downloadable from My Oracle Support to fix this problem.
See message translations for ORA-02291 and search additional resources.