What may cause "An illegal choice has been detected. Please contact the site administrator"?
I would say that this means that the user has selected term 423 for term_node_tid_depth_1 and 427 for term_node_tid_depth, but those values are not actually options in that field.

Usually the way this happens if you have custom code that sets the field value to an invalid value, or if it changes the field and removes some of the possible allowed values.

You say it doesn't happen when you are the admin, so I would guess that the admin has access to all the options in the field but other users are just seeing a subset of the field values, but somehow are still setting the field to one of the values they aren't allowed to see.

Do you have any custom code or are using any unusual field or forum related modules?