The issue appears with users that are not member of any group but active. this is not allowed in omero 4.4. those users will not be able to login. unfortunately one cannot change the user groups of such an account in Columbus, it will throw and internal server error again, which we cannot fix as it is within the Omero server. In order to solve this problem one needs to remove the 'user' group from the account in the database to make the user inactive. once this is done it can be edited again and groups can be added, as well as the account marked active again.
here's a bit of sql to list users that are only member of one group which, if it is the 'user' group will show the problem:

psql> select e.id, e.omename, g.name as groupname from experimenter as e
left join groupexperimentermap as gm on gm.child=e.id
left join experimentergroup as g on gm.parent = g.id
where e.id in (select child as c from groupexperimentermap
group by child having count(*)=1);

id | omename | groupname
-----+---------------+-----------
1 | guest | guest
103 | alain.baulard | user
(2 rows)

Now with that information you see that the user with id 103 is active but not in any other group. You can then delete all entries from the groupexperimentermap table for that user.

psql> delete from groupexperimentermap where child=103;
DELETE 1
Once this is done, one can edit the user again in Columbus and activate her/him and add groups.