Survey for RT 3.4 / Rights to change Custom Fields

I’ve cleaned out some things in the Survey-code found here: http://rt3.fsck.com/Ticket/Display.html?id=3317,
now there’s one finally one problem left I cannot resolve (RT 3.4.1 on sarge):

When I submit the survey as “not logged on”, it tells me "Could not add new custom field value. Permission Denied"
In the log I have one "[err]: Couldn’t load from the users database."
What fails is ProcessTicketCustomFieldUpdates(ARGSRef => %ARGS) with the above error;
Shouldn’t it be irrelevant which rights are given when the Ticket is loaded with $RT::SystemUser ?
I gave any right I could imagine (test-box) to Everyone "SeeQueue, ModifyCustomField, SeeCustomFields, even SuperUser), still the same…

Survey.html uses $RT::SystemUser to load the ticket, displays it fine and I can also add comment.
But changing custom fields doesn’t work for some reason, maybe there’s someone around to give me a hint…
Or is it maybe broken, i.e. somehow impossible to change custom fields with the ModifyCustomField right ?

Michael

Maybe for those interested:
I save the survey into a comment now and use ExtractCustomFieldValues.pm to fill it into the appropriate CF’s. A bit complicated though but it works fine.
Still strange that I can add comments but not change CFs with using the RT::SystemUser loading the ticket but couldn’t sort out with my limited programming skills…

Michael