Cannot add users to New or Existing Category (but adding groups is OK !)

[Ian Wolstenholme]

If I try to Add a New Category called "test" and then attempt to add named
users to that category, on trying to Save the category I get an error
message "An error occured while completing the operation.

Very briefly in the Bar at the bottom of the page (where it normally sayd
"Done" I see the following
"http://dev3/ProjectServer/Admin/sec_svr_category.asp?nCookieLong0=9737165&nCookieLong1=771804162"

If I do not add users I can create the category.

Assigning Views, Models, etc to the category are OK.

I can even assign groups and save without the error. I ONLY get the error
when I am assigning users.

If I modify a user, I can add the category to that user with no problem.

So why do I get this problem when trying to assign users to a category
whilst either modifying or creating a category ??

Thanks

Ian