Issue with BEFORE INSERT trigger

[gtcol]

Hi all
I've caught into strange situation here. Attached query works just fine in our Dev RAC, but just won't in our Prod RAC. We have a BEFORE INSERT trigger that takes nextval from a sequence. I've attached the query, trigger, sequence and table DDL. When I run the query it gives me
'ORA-01400: cannot insert NULL into ("USER_T"."USR_LLG_DETAIL"."LETTERID")'.

Any hints, suggestions are appreciated.

Thanks
gtcol