interesting use of a check constraint to make a table read only;
could there be a trigger on the table that is also preventing your modifications? depending on the loging in such a trigger, it might explain your situation.
help us help you! If you post a question, make sure you include a CREATE TABLE... statement and INSERT INTO... statement into that table to give the volunteers here representative data. with your description of the problem, we can provide a tested, verifiable solution to your question! asking the question the right way gets you a tested answer the fastest way possible!