I have been mulling over this issue - one I have never encountered before and that only ocurred after my first attempt at using the Edit-This-Page package.
The error thrown by acs__add_user :
[28/Nov/2003:18:09:04][2857.81926][-conn2-] Error: Ns_PgExec: result status: 7 message: ERROR: -20001: membership_rel violation: Invalid object types. Object -2 (application_group) must be of type group Object 848 (user) must be of type person
Looks to me like corruption of the acs_objects table, so I wanted to check certain behaviours of ETP in case I have misused it.
I mounted ETP on a first level site node and expected to be able to add subnodes - which I did. What I did not expect was to be able to click edit-parent-page and be given a page relating to the Main Site node. However once I found that I could do this I added a number of subtopics to this and edited those as well.
This had the effect of adding new first level site notes which appeared in site-map as new package instances of ETP that had not been mounted using the package manager. They seemed to behave OK until I encountered the user registration problem.
So what I am wondering is whether in fact this should not be happening and that adding subtopics at this level has mixed up the object_type references so that the database thinks that I am trying to add someone as a user of an application group.
For info, the acs__add_user failure did write to the server log but did not report failure to the browser, so for the user it failed silently.
Any help and guidance gratefully appreciated. Meanwhile I will rebuild from a fresh install, re-arrange my ETP structure and try a DB restore.
Regards
Richard